Is it unethical to accept extra time on exams when I still do well?
Absolutely not. If you meet the full requirements set up by your University, it is perfectly ethical to use all of the resources made available to you. ADHD is a real medical condition and medical experts and policy makers have decided that special test accommodations are the fairest solution. Accommodations like this are based on medical decisions, not whether you are doing well in the course. Take the extra time if you want it. There is no need to feel guilty about doing so.