So, reading through I think it's fair to say you recognise these two children are already traumatised and will feel rejected/abandoned if you disappear entirely, but you are worried about how to balance that with extracting yourself from their father's control and domination as understandably you don't want to leave him but have nothing change.
My observations and practical suggestions...
I think because his control has basically been all you've known for your adult life it is very difficult right now for you to be able to imagine him being a presence in your life without still controlling you. For instance, you've mentioned a few times that you're worried if you have a set day where you pick them up to bring for tea / he drops off for tea that he will start pushing boundaries with phone calls saying he's running late so can you do school run etc. To me, the solution to that is very simple - you say no and don't enter discussion.
Right now, you live in the same home as them and you are a bit cowed from years of his domination. It is very difficult to say no to school runs and bedtime routines and all the rest when you are living in the same house (and you will also face consequences in your home for not submitting).
Whereas when you live separately your "no" is powerful and something you can enforce. He won't be living with you to make life uncomfortable at home if you say no, the children won't be living there for him to abdicate and get away with it.
Does the prospect of saying no to him frighten you in some way? Like when you imagine the hypothetical scenario of him phoning and trying to get you doing random school runs once you've separated, does it frighten you to imagine yourself simply saying no and sticking to it? Can you just not even imagine having the power to say no?
The big difference once you are living separately and on the way to divorce, is that you are going to have power and control you have not had before. It will take you some time to learn to navigate that given the background and that it will be new unfamiliar territory for you, but you will have power and with a bit of time you will be comfortable with it.
So even if you choose to have limited set fun visits from the children for their benefit not his, because you unlike him are a decent person (a framing that will also make a difference to how you feel about it, instead of viewing it as him winning or whatever), it won't be how things are now. It won't be a continuation of the situation you are seeking to leave, it will be very, very different.
So you can decide what your boundaries are and you can enforce them. If he tries to push them you say no and stand firm. If you initially struggle adjusting to having that power you can write them down for yourself and write a short letter to yourself reminding you that you can say no, that you have power now, that any contact you have is for the welfare and benefit of the children not him, etc. (Written reminders can really help in moments where we feel overwhelmed).
For instance, those boundaries might be no school runs, no medical appointments (other than an emergency while with you obviously), no parents evenings, no regular sleepovers/no sleepovers, no proper parenting basically. But that you will see them regularly and consistently from a relationship point of view to spend time doing fun things without the parenting role - and then the types of things you're happy with (coming over for tea a couple of times per month, a fun day out once a month, a sleepover in the summer, birthday cards, whatever).
Model it on a family friend or aunt type relationship - maybe write yourself bullet points of what you won't do anymore and what you will do. That will keep it clear in your mind if he tries to push your boundaries and also enable you to be clear and consistent with the children (both in how you behave and how you communicate or answer their questions) so they feel secure from knowing where they stand with you.
You don't have to cut them out of your life to cut off their domineering father's power over you. Although I appreciate it's probably quite hard for you to visualise that now, that's just because his domination has worn you down - it will be possible and you will be able to see that once you have some distance from him.