I just think the HOA is abusing their authority, even if there's a signed agreement.
Not at all abuse of authority. How is enforcing rules, you and everyone in that community agreed to when you purchased there, abuse of authority? If you thought that was unreasonable, you shouldn't have moved there to begin with, since I assume you had the by laws to review and sign off on before you could purchase.
You had the choice NOT to move in there if you wanted to keep commercial vehicles parked in your driveway. Likely it wasn't a thought at the time when you moved there, but now it is a consideration and suddenly the rules aren't fair because the are not working for your situation.
We didn't get the van lettered the first winter we got it so we could park it in the driveway and plug in heater. We slapped on magnets every morning and took them off at night. Got lettered in the spring and then found a place we could keep it once we did. We had a dry cleaner in the development. Parked the van in the driveway everyday. You could see the clothes hanging in the back windows, but since the van was not lettered, it was within the guidelines.
Commercial lettered vehicles were permitted if they were parked in the garage, but our work van was too long to fit in.
People choose to move into these types of communities because they don't want to see a big lettered vehicles lining their street as well as a host of other things that are nuisances in unregulated communities.