5 yo having difficulties with potty training

So it's not as bad as it sounds, but my son is 5 yo and he still struggles with potty training. He knows exactly what to do, and has no problem doing his business in the toilet (especially for #2), but he gets so hyperfocused on one thing or another that he forgets to go.

We've tried to use positive reinforcement via stickers and a treat as a reward. For a short while, we had a sticker chart as well and he could pick the sticker he wanted and put it on the column for the day of the week it was. That sort of fizzled out, though.

We've tried to use negative reinforcement like going to bed 30 minutes earlier than usual or getting a toy/book taken away that he can earn back if he goes potty in the toilet 3 times in a row without going in his pants.

We're getting back to the point where we have to start reminding him every 20 minutes again. Nothing seems to stick and he's getting to be too big for the trainers he wears. We've done potty training books, as well, which helped in the beginning, but now it's just getting him to stop ignoring the feeling of needing to go potty that's becoming the biggest challenge.

How do you guys handle potty training your toddlers and pre-k-aged kids with ADHD?