Our previous dog had a hard cyst the size of a quarter on her chest, it never changed but we had it removed and it was benign. She got two more as she aged but we never had them removed.
Our cat has a cysts that seldom changes, but went away once when the vet gave her prednisone for flea allergy. It then grew back to old size, we decided not to remove it. Maybe in the future if your dog has to go under sedation for something like teeth cleaning or whatever, you can have it remove, that would seem more practical if you think it is benign.