My eldest was such hard work at that age, you have my utmost sympathies.
It won't be like this forever, I promise. FWIW DS grew out of this stage when he was 4, but it felt like forever when I was in it.
I just kept trying things till I found stuff that worked.
One thing that I remember working surprisingly well was to give him what he wanted in fantasy version, instead of saying no.
So, if he wanted to eat cake he wasn't allowed, instead of saying no and getting into a battle of wills, I'd say something like - oh that looks amazing, wouldn't it be great if we could eat cake all the time, and chat away about cake while distracting him from the actual cake. This worked surprisingly well!
It might not for yours, but keep trying stuff.
For teeth brushing, turns out he hated having it done TO him. It helped if we brushed out teeth together. A sand timer helped. Playing songs while we brushed helped. And if I was feeling especially brave, letting him brush mine while I brushed his. (Not always successful, sometimes painful!)
About sleep, I accepted he's a night owl and moved his bedtime later. MIL was horrified lol. But I didn't see the point in both of us sitting in a dark room with him not sleeping. Your compromise on that one sounds good.
I remember I found some of the advice in this book useful.
https://www.worldofbooks.com/en-gb/products/calmer-easier-happier-parenting-book-nol-janis-norton-9781444729924
Ultimately though, what saved us was he grew out of it. And yours will. Until then, it's a case of muddling through and doing what you can to make things bearable, while repeating the Mumsnet mantra "this too shall pass!"
FWIW, DS is the easiest teenager these days!