my sympathies. my almost 9 yr old DS, diagnosed ASD 18 months ago, is similar, and we've had times of considering an outright screen ban, but like you pulled back from that because when it's going well, it really does calm him. unfortunately, it's during games with other people, online, that he can't cope with, if someone kills him/steals his stuff, etc. when he was assessed he was in a You Tube obsessed phase (making his own videos) and it was all he talked about during the appointment, he brought everything back to that (his one question to the psychologist was "will you subscribe to my channel?") so it was at least totally apparent that this was/is his area of special interest, and as such it was suggested we let him pursue this... but it's a fine line to tread when it also makes him very distressed sometimes, can contribute to terrible behaviour to the rest of us, especially his younger brother, and can interfere with family life, normal routines, non-screen activities that I deem also essential to his and our wellbeing...
I don't have the answers but what's working for us is an unwavering timetable. no devices go on before 7am (or they'll both be up in the night). DS's PC doesn't go on till 9am (so on school days he doesn't get it till after school). between 5pm and 6pm, everything is off. they can watch a film if they wish, on the family TV, all together, but more usually we've been playing board games. this is a new development and it took a week or so for them to adjust (younger DS is not autistic, he just wants his own way) but now DS1's devices go off and he happily comes through to have dinner and play. in term time this is when homework is looked at. we use Kidslox on the ipads and a Microsoft controller thing on the PCs and xbox. then after, he can have 1 hour only doing the thing that caused me to to instigate this rule in the first place - Fortnite. I loathe it, and we've had flat-out bans at times, but at the moment he's dealing with it. it's fully supervised, extremely limited, I do not bend to pressure for more (this more than anything is DS's crack). and it's going ok... my partner supervises and DS enjoys that, he talks through what he's doing, wants to share (and he gets knowledgable feedback rather than "a that's nice dear" that I would give). bedtime is a strict 7.30pm on school nights, 8.30 the rest of the time.
my DS is also very far from self-regulation. but after a particularly awful meltdown one time, when we talked later, he did concede that some things he just couldn't handle.
also, I find this terribly hard, but I don't argue with him (or I try not to). at times he hates the rules, and doing pretty much anything else that has to be done, like go to school, get dressed. he's persistent and it drives me up the wall but I have to just. not. get. into. another. discussion. have you asked this already? yes. did I answer? yes. well there you go. the end.
the struggle is real. technology is definitely a double edged sword. but it's enabling him to have friends at school, because Roblox, Minecraft and Fortnite is what they're all into. do I wish he was bookish and sporty? sure. oh well. one day he'll learn to code and then... 