PDA - pathological demand disorder is worth looking up. It shares characteristics with ODD but it’s not as hopeless a prognosis. I agree with a pp that ODD is a very serious term to throw about lightly. (’m not saying you are, op. But that’s a hell of a burden to lay on a parent without being absolutely certain it’s correct and then signposting meaningful care)
Some people prefer to rename PDA as Persistant Drive towards Autonomy which, imo, is a very helpful way to conceptualise it. And can help you align with your child’s needs instead of being the antagonist.
My ds has a strong streak of demand avoidance (as I have slowly uncovered, have I). Things that help include rolling transitions (eg 5 more minutes, do you need a little more time ), indirect future focusing phrases (I think xxx is on tv tonight … said shortly before a fun activity is ending, to get him thinking about later rather than stuck in the now that is ending.) and a heavy emphasis on regulating sensory experiences.
There’s a 5 point emotional regulation scale a lot of therapists use. Our dc can be hovering around 3 and 4 so it feels like they blow up to a 5 over very small provocations. If you can work on lowering the baseline stress level to a 2, it makes a massive difference because it takes longer to climb to a 5 from a 2. But our dc are under so much day to day stress that small demands can be triggers
My ds is autistic which means that rules (created thoughtfully and sparingly) can be very comforting and help to hold boundaries. It’s easier to follow a rule than meet an arbitrary demand even if the rule looks to us like a harder demand.
Your rule about hitting is a good one- my advice is to keep that as a neutral statement of fact, rather than thinking of it (or expressing) as a punishment. The loss of self control comes with a heavy backlash of self loathing already.
Help your dc learn the socioemotional skills of repair and rebuild. Model apologising, stopping, re-evaluating, reconsidering. Especially with an adhd mindset - there are usually at least 6 ways to see any issue!
ADDitude.com is an absolute treasure trove of information and resources.
The podcast Good Inside might be worth a listen too (it might have been a book). Not so much on adhd/odd/pda but just as a parenting philosophy.
I personally have mixed feelings about Russell Green’s The Explosive Child but it is a good starting point.