Also very reactive ADHD kid (no ASD dx but traits) here. We live somewhere PDA is not diagnosed at all, though I do find some PDA resources helpful. Stimulant medication has been absolutely like magic. I have a very happy, very autistic-leaning 7yo who has about 3 interests and 2 friends (one of whom is his sibling), can be rigid and can get hung up on things being done right and still doesn't recognise that he is a child and adults have a different social status to him, but we don't mind this, and he seems to begrudgingly accept that it is the way at school so we are all (tentatively!) surviving for now.
I would be cautious about walking on eggshells. I do understand this, and sometimes it makes sense, but I second another poster who said sometimes avoiding triggers just leads to you playing some kind of reverse whack-a-mole where you don't even have a mallet - more triggers will always come up because it's not actually that the thing itself is an especially damaging or difficult thing for them, it's just that they are feeling like an eggshell and any small force can cause them to crack in that way, and plus because it can be self-feeding and grow in the same way that anxiety can. The thing is that it is so costly to the rest of the family to do this, especially siblings, and it is not always actually helpful to the ND child to have it reinforced that everyone else has to bend around them at all times. It is true that ND children often need accommodation that NT children may not, but I think it's important also to uphold the rights of e.g. other children. For example, even when DS2 was exploding a lot of the time, we always had a rule that he could decide what he did, but he could not control what other people do. He could make a request, and the other person could choose to follow it or not. The only exception we would make was where he was e.g. asking for space (even if he was doing it in a non-ideal or outright rude way) but if his request was totally unreasonable (e.g. if you colour with me you can only use the pink crayon) then we would lay out the actual terms (You can decide if it's OK for DS3 to sit with you and colour, but if you're colouring together, then he can use whichever colour he likes. If you're colouring apart, you can divide the crayons, but you need to give him more than one.) If he was finding it hard to cope with this, then we would take him away from where he could cause any damage to e.g. the crayons, DS3, the drawings and try to co-regulate with him to help him calm down and work through it.
RSD I think is a particularly sticky concept - I think this can be helpful for adults, but for children I don't know that it is especially useful, particularly the way it gets used in some online ND parenting spaces. The D does not stand for disorder BTW, but dysphoria, which is a more temporary experience than a permanent thing. The term was originally coined (by Dr. William Dobson) to describe an experience many people with ADHD relate to, not a standalone diagnosis, and it is not considered a diagnosable condition, although there is research on the concept of "rejection sensitivity" (which is not unique to ADHD).
It is most likely that a combination of repeated experience of social rejection (making it associated with painful or sometimes traumatic memories) plus emotional dysregulation (which means the person's emotions don't have a "volume control") is what is behind the adult experience of RSD. I feel like the way it is presented online is more like something which is inbuilt into a person and will always be present, and I just do not believe that is likely to be the case. It reads much more, to me, like the way that e.g. someone who is dyslexic and struggles with reading and spelling might find insults relating to their intelligence to be especially difficult because it brings up painful memories or touches on an aspect of self-esteem which is especially fragile.
ie, it is a completely valid and understandable response, particularly in an adult or older child who has experienced some combination of peer rejection, bullying, harsh parenting/criticism from other adults, repeated job loss, etc, often in the context of not knowing they have ADHD and not knowing how to change those patterns - I get that, it's a really horrible thing to experience, it likely mounts up to almost a sort of trauma response and automatic, self-protective reaction. However, I think it absolutely is something which can be overcome, this would be twofold - basically the emotional regulation skills on the one hand, to try and help the child/person to recognise when they are getting pulled into an overwhelming emotion and then some self-soothing/self-regulating strategies to ramp back down or even just something as simple as giving it time before taking any action. Then the other side of it would be seeking out and supporting any positive social relationships/experiences, working on social skills and communication, ideally within the environment with other children, and ideally increasing support for the child in places they will have social experiences (e.g. school, clubs) in order that they are less likely to end up having an experience where their behaviour upsets other children.
Where I think the way it's talked about online can be unhelpful is that children with ADHD often find it extremely difficult to hear criticism or feedback or accept consequences, and if this is put down to "RSD" and parents are made to feel guilty or that they should not exacerbate this, it can make you feel as though you can never give your child feedback on their behaviour or hold them accountable for anything and that is not usually helpful. It's nuanced obviously because on the one hand no, it isn't helpful to be constantly piling on loads of criticism and consequences for things they might not actually be able to help, and also we're human so it is easy to fall into patterns of complaining about things you notice which are irritating while forgetting to acknowledge the good things you are seeing because you are too stressed to notice them. These are things which it can be helpful for parents to be aware of as patterns which we might easily fall into and want to avoid. There are strategies to follow for all of these things. But OTOH if you are focusing on one thing at a time which is within their grasp and you are getting plenty of positive warm supportive interaction in, then IME it can actually be really helpful to use a very minor, token, boring and temporary consequence to mark certain behaviours, even if only as an exercise in learning that actually, you will make mistakes and it is not the end of the world and nobody thinks you are a terrible person because of it, and your parents can both love you AND be frustrated by something that you do, that your actions have consequences but you can handle them and you always have a chance to try again. That's healthier than keeping your own emotions all hidden away because you are worried that it is going to cause damage to your child or spark off a meltdown. Actually weirdly and tangentially, although it's fiction, a bit outdated and wasn't that realistic when it was written, I think the Tracey Beaker books, especially the character of Cam is quite a good example of a way that adults can speak to children expressing frustration but also love at the same time. I re-read them recently and found it stood out to me - everything online seems to be so divided where it's all either robotically compliance-based (not my thing) or so passively accepting that you are basically supposed to lie down in every puddle because you must not allow your child to experience being wet. I lean towards this one mostly, but I think it just sometimes well-meaningly goes too far. Like if I can't control the rain, then I don't need to protect my child from every metaphorical puddle, either. And honestly they don't melt. My own child's doctor was the one who told me to stop rescuing him so much and I did pull back a bit and I feel like doing that helped me to recognise when he does need me to go in and advocate for him, and when I just need to let him have an experience, even if it's not a nice one, and trust him that he will deal with it in his own way.
I do also really like the Ross Greene stuff in terms of giving a bit more structure to low demand, although I don't think it works for everything. I find listening to podcast interviews with Dr. Greene to be more useful than the FB group which is a bit too rigid for me and swings wildly towards "plan C everything even if it means plan A for everyone else".
Any PDA related stuff which talks about autonomy is fantastic - especially where it gets into the nitty gritty of how to handle/balance when the child's autonomy clashes with another person's autonomy, because although their autonomy is important (so much that I genuinely think this is important for every child, NT or PDA) it does not trump another person's autonomy, but equally nobody else's autonomy trumps theirs.
The Linda K Murphy books are absolutely top notch. I would honestly buy these for school if you think they would read them. Some of the best and most practical without feeling the need to sneak in criticism for the opposite approach.
Something explaining an idea of "levels" of nervous system reactions - Mona Delahooke or Robyn Gobbel are the best for this one, I think. And possibly Stuart Shanker. DS2 does "baby voice" when he is getting overwhelmed as well. I do think it is incredibly helpful to be able to recognise when they are getting close to their limit and to take action before you get there if possible, rather than keep pushing on until they well and truly melt down. If something is important for a sibling, we always try to either make sure that both of us are there, so that one of us can leave with DS2 or take him out for some space if necessary, or we don't take DS2 in the first place. Now that he is on medication this is much less necessary, but before he was medicated it would often be the case that we just wouldn't take him. I felt it was important for DS3 to be able to experience things without DS2 becoming the focus of the only parent's attention. This does have consequences for me - I can't work currently, and we are fortunate that DH's job is fairly flexible and he can WFH as well, which helps a lot. We don't have family nearby.