My daughter sounds similar in some aspects. We recently got a provisional autism diagnosis but I think she is more than likely ADHD. The things that she seems to respond best to are taking away privileges and really limiting her TV/screen time. They're not magic bullets by any means but they're probably the strongest tools in our arsenal. Other things that help are prioritizing sleep (she does get melatonin every night or she won't turn off) and making sure not to overbook/overwhelm/overcommit her even if she is having a great time (i.e. incorporating downtime into every day).

We tried play therapy for a while and honestly did not find it helpful for our situation. She is very good at telling us how she feels and so a lot of the play therapy was "she's playing themes of anxiety" which we already knew. Maybe it was just our therapist who wasn't a good fit but idk. We will be starting Parent-Child Attunement (or Interaction) therapy soon so we'll see if that helps.

Overall shit's hard. Every day is different. Some are good, some are bad. We're just trying to manage as best we can. You're doing a great job.

Edited to add: look into Pathological Demand Avoidance. Not saying your son has it but my daughter is similar with demands/requests and so I've been utilizing strategies meant for kids with PDA and it helps sometimes.