It sounds like you already have a great routine for getting it done, maybe you’re putting too much pressure on yourself by trying to make it fun! I don’t know anyone, ADHD or non-ADHD who considers brushing fun.
If you do want to try to make it fun, maybe listen to music while brushing, use a light-up brush, or promise yourself some kind of reward for after you brush or plan a bigger reward for yourself for every month or year that you stick to your routine?