I know we need to be firmer with her, in the moment it has always felt like a case of “pick your battles” but we obviously aren’t having enough battles so now the boundaries are too blurred.
Yes. This is common if you come to gentle parenting from a position of "I don't like conflict because my only experience of conflict is scary and unhealthy" and/or "having boundaries is mean if it makes people [my child] upset" and/or "it's my job to alleviate as much discomfort from my child as necessary; I should weigh every incident against the necessity [safety, etc]"
Most gentle parenting type resources are aimed at parents who are too controlling and too authoritarian. So advice about picking your battles, only having rules when you need to, using less conflict, loosening the boundary, handing over control - these will not be calibrated for your starting point if you're coming to it from a place of being too permissive or too people-pleasing or too martyrish.
Then it's difficult to find advice that you feel comfortable with because a lot of the advice coming from "You're too soft, you need to do XYZ" will be from a rewards-and-consequences mindset which if you've spent loads of time reading about the downsides of this will probably feel unsatisfying. Although I will say that although I lean more towards gentle parenting in general, when I was getting so stressed that it was causing me to shout and scream I realised that doing this was much worse than a calm, controlled, clear, non-scary consequence such as losing TV time or similar, so if you need to dial back from the gentle scale and lean in to reward and punishment? Do it. It doesn't mean that all the benefits of the other bits of your parenting will be undone. You can choose this as a short cut just as you would sometimes serve chicken nuggets and understand that is acceptable when the whole diet is balanced, just as sometimes you will have whole weeks where you only serve convenience foods [you're ill, you're having a kitchen renovation] and understand that it doesn't matter because you're working on the resources that will allow you to build a more balanced diet.
So building a more balanced parenting:
Understand that consistency is about always having the same boundary, not necessarily about always having the same response. If you sometimes respond to jumping on the sofa with a threat of punishment and sometimes respond by talking her down and sometimes respond with distraction, that is still consistent. If you think that the only way to be consistent is to have the same response but your chosen response is to talk/explain/redirect and on some days you don't have the energy so you pretend not to notice or do it half heartedly without following through, that is not being consistent and she will be confused at what your expectation is.
Understand that being a "fun parent" is not about saying yes all of the time. You actually build an incorrect assumption in here. If you have in your head that the rule is "we don't jump on sofas but I want to be fun so let's do it today!" and today is happening 5/10 times, then your rule isn't clear at all. In fact what she experiences is that the rule is that jumping on the sofa is fine, and the days that you say no she thinks you're just being mean for no reason. (This can also reinforce an incorrect belief in the parent that boundaries are mean.) It actually leads to the opposite perception - you need to have some clear rules that you occasionally (like, 1 in 20 times) deviate from. Not always deviating from the rule because it doesn't matter today but the time it does matter, it seems unfair to her.
Turn pick your battles on its head - instead of thinking does this matter right now and therefore saying no and dropping the battle too much of the time, ask yourself will this make my life easier in future? And if it does, that's a great battle to pick! It's just about adjusting the lens through which you view and make these in the moment decisions.
Read/listen to Janet Lansbury about accepting difficult feelings. If your feelings were dismissed in childhood you can feel an incredible sense of responsibility for your child's emotions, which is misplaced and will make it very difficult to make decisions which cause her to suffer in the slightest way unless they are very well justified. Again adjust your lens here. Your child is responsible for her own emotional response. You are responsible for allowing her space and co-regulation if necessary, and language to explore/process those feelings, you are responsible for preventing harm caused by expression of those feelings, you are responsible for not maliciously causing upset, but it is not your responsibility to prevent any upset feelings at all, and it is not helpful to try and do that. They will experience sadness, loss, disappointment, frustration etc in life. They start learning the tools to deal with those feelings by being allowed to experience them.
The Whole Brain Child is also good here.
Still Awake is a great book by Lyndsey Hookway about toddler to teen sleep. In terms of boundaries and patterns that you may want to change. I would highly recommend it.
I have heard Parent Effectiveness Training is good for a better understanding of how and when to set boundaries without being overly concerned with punishment (which is not the only way to uphold a boundary). I haven't read it though.
You can follow all the other great stuff in gentle parenting even if you come from this mindset! Just keep in mind that most advice around boundaries, expectations, rules and how to keep them probably isn't aimed at you, and won't help you calibrate.