The only thing that worked for me (also ADHD) is to have fixed study sessions, typically in the morning ("Kanji before breakfast") or during my train commute.

Beyond that, I wouldn't worry about backlog too much. I use a custom deck that gives me my maximally doable number of cards per day and completely ignore the rest. It's not like the actual repetition frequency is critical - many people change those settings quite drastically and still manage to memorize their cards fine.