You say "some people have views on adopting with a birth child, especially as a single carer" in a way that seems slightly dismissive, and I'm interested in this. Who has said that? And why do you feel you don't need to listen to them? I'm not saying you are wrong, but throughout the adoption process I've found it's easy to dismiss the concerns of other people if they don't quite agree with your position, only to find they actually had quite a valid concern, or lived experience or whatever.
I speak only as quite a headstrong person myself, who was told quite a few things that, upon reflection, were right, and had I listened or at least understood their perspective, it could have given me some valuable insight.
To be considered for adoption, sometimes I think it's funny because you need to be proven quite successful in many areas- socially, financially, emotionally, intellectually in order to get through the process. However, the things that often go with this success (determination, sometimes pig headedness, rising above all challenges, belief that one knows best and powering on through like a bulldozer etc) are the ones that mean we ignore, dismiss or diminish advice, especially if we don't feel that it fits our world view. You sound quite successful- to be a single parent and financially and emotionally comfortable enough to be considering another child, to be independent to such a level, demonstrates your success. Those characteristics are to be admired. But I would also suspect you have had to fly in the face of people who, possibly brazenly, possibly subconsciously, suggested your life would be easier or better if you were partnered. And you might be quite (rightly!) be used to saying sod off, I know what's best for me. And that's fine, for the most part, because adopters need to be ballsy and pushy in order to advocate for their child and themselves. However, I do have a few thoughts.
I remember being outraged by a social worker who said I shouldn't even think about adopting siblings who had not lived together previously because they would always be anxious around each other, fight and never truly settle. Ultimately, I genuinely believe we were right to adopt the kids we have... And yet their relationship with each other remains one of the defining factors of their lives, characterised by anxiety, yet deeply misunderstood by the professionals working with them because a sibling bond and attachment is not a particularly common one to work with in the same way that parental attachment is. It is only after nearly 2 years of work in therapy that anyone has even mentioned their relationship with each other, which we considered to be one of the fundamental parts of their lives. There is precious little support about kids in our situation, and so we have had to make do and mend, adapt advice about blending step families with trauma informed practice and adoption practice with what we know about them etc. It's a whole thing.
And so, when you have people saying things like that to you, I would always consider the provenance of their opinions. Are they experienced adopters, social workers, adoption specialist therapists, or an adoptee themselves? Or is it some random acquaintance who has never met an adoptive family, who wouldn't understand the nuances of the situation and thinks all adoptees should be grateful for a family (lolololol)? I'm not saying the experts are always right- my kids are a proper little team now, and I have no regrets about it, but I would say that, looking back on it, that old battle axe social worker had a point. We glossed over it, dismissed it as an outrageously inhumane thing to say (siblings! Together! Family! Reunited!)... And yet her concerns held water, and she was right. Our lives, and our boys lives, are not always made easier by a very complex sibling relationship that has little precedence in even the adoption arena. Our lives are richer, more satisfying, more energetic, funnier and infinitely better because we have both of them, but it is not easy, and I wish I had looked past my own outrage to look at what her concerns were in order to make a properly informed judgement about the level of support that was available for this specific issue. It has been difficult to be continually dismissed by those who know nothing and say "all siblings do that", and to be dismissed by those who know something, who say "the primary caregiver bond is the only thing that matters" and those who know a lot... Well, when I've found them I'll let you know!
So what I'm saying is, you know your situation, but don't dismiss out of hand concerns that people raise. It may not be kind in the way they say it, or feel pleasant, and might not be a reason to stop, but it might ultimately be a reason to pause and consider. That battle axe social worker is the reason we felt able to handle it when the kids were physically fighting so badly that they bled- we had done such a a deep dive into why we thought she was wrong, researched how little information there really was about this and then formulated our own plan with the therapist to reduce the emotional and physical harm that we felt as prepared as it was possible to be.
I'm happy to pontificate further about adopting with a birth child, despite not having done it myself (because why not, what else is the internet for, eh?) but this is what I mean about checking credentials. My experience is only relevant for certain aspects of your story, and whilst you could draw upon it more than, say, someone who had a new baby sibling, there is less to be gained from it. But you might find a nugget in my comments that is applicable to your situation.
I would strongly advise that you go to a few adoption events where you can speak to a social worker about the process, and about the implications adoption with a birth child might have for you, the BC and the AC.
However, off the top of my head, these are the questions I would be asking myself:
How does my BC deal with children who show unwanted behaviour? Would that change if the other child was in their space or in their territory?
Could my BC handle not being the centre of attention for long periods of time?
Would BC feel rejected or left out if I had to take AC to appointments, introductions, transition sessions etc? Especially if they weren't invited?
How does BCs story and narrative compare to ACs? Are there similarities that might make them worried or scared?
How much of a support network does BC have, and would you be willing for them to use it extensively? (They might have a great relationship with grandparents but would you truly be happy if they were significantly dependent on them for emotional support if you were all tied up with AC?)
How are you going to prioritise your relationship with BC? Especially if things are tricky with AC? I'm talking practicalities here- is your support network able to cope with looking after tricky children? And even if they can, is one dedicated bonding activity a month with BC really going to cut it if the two of them don't get along and you have to focus on AC most of the time?
Are there any factors at play with BC and wider family? I.e you say you are single, but there must be a BF in the narrative somewhere. So would this cause any issues either way- either jealousy of an absent father from AC (why does your BC get to go with dad every weekend when I can't, why don't I get to have a dad) or from BC (why does AC get all the attention for not having a dad, I wish I didn't have one/didn't have to see them/am jealous you and AC spend time together when I'm with my dad) etc. Kids aren't always logical, and also are incredibly tactless. I'm in a SS relationship, and my two have recently started yelling "daddy, save me!' whenever they jump off the sofa onto the floor, then saying "you can't save me, you're not the daddy!', because they saw a cartoon of a sinking ship somewhere on their tablets with a daddy who rescued his kids. (Something to do with the the titanic) The therapist reckons they are processing abandonment, I think they find the very idea of me being a daddy hilarious, my mother is aghast that they aren't more sensitive and realise they 'don't have a daddy", which they do. And don't. And they have 3 mummies. But only two who live with them. Actually only 1 because I'm mama. Queer adoptive families can play fast and loose with stuff like that I guess. We have a dark sense of humour, so I bought a big fan that says daddy and I fan myself dramatically saying 'but my fans say I'm daddy!' and then rescue them anyway. However, this mainly works because neither of them are more sensitive about it than the other, we all have a weird sense of humour, and factually, there isn't a daddy in the house, it's all play. It wouldn't work if one of them was actually seeing a daddy every weekend, if you see what I mean.
Are you planning on a gender difference? Why or why not? Worth putting your thoughts in order.
Does BC have any other factors at play that might disrupt equity between them (SEND, confidence issues, dual nationality, step family, physical strength, health concerns, friendship issues etc) that might make them significantly different to AC?
I'm sure I've heard of support groups for BC actually .. that might be worth researching, but again it's about recognising your BC will be given an identity as the sibling of an adopted person, which has a specific set of challenges, and a lifelong commitment you are making for them to be a part of this minority group with possibly not very widely recognised (and sometimes socially inadmissible) feelings. Your kid might feel quite guilty about saying they don't like their adopted sibling much because all they do is ruin their stuff and argue with them, or feel jealous that they don't have your attention more, or whatever problems siblings usually have. But the problems have the potential to be exacerbated because.... Well, because trauma. They might find it difficult to admit these feelings to you because you are the one driving the adoption forward, but also because they might be excited about having a sibling, and love them very much, but not like how they do xyz. I'm not saying this is every experience, because there are many adopters on here who have BC and AC and it's a breeze, but I do know of some families who have really struggled with this, particularly if the AC is old enough, and has agency enough to cause problems for BC. 8 years old and a 1 year old sounds like a decent gap, and tracks well in that at 8years old, BC will be 16 and firmly in the next stage of life with no comparisons. An 8 year old and a traumatised 5 year old could be more tricky, for example, because good lord 5 year olds can be really annoying, and a badly handled broken Lego set at an inopportune moment has the potential to trigger a lifetime of resentment at the injustice of it all.
You have a lot to think about, and as ever, I would advise going through these boards and seeing what challenges other people have brought up, what reflections you can make about them and use it as a springboard into understanding your own situation. You won't be ready yet, because you haven't been through the process, but you will only ever know if it is the right choice by starting.