Had a similar thing with my childhood cat. Vet was insistent on 3 monthly reassessments (despite being told it would likely be for the rest of her life) which were $100 for a 5 min “yep all good”. Also insisted the only place to get the medication was through that vet with their prescription.
Moved, new vet seemed appalled and said he needed to see her once a year to reassess and gave us a wholesale pet medication website to order them from. Cut the monthly costs from roughly $200 to about $80.
I understand that it’s the equivalent of a private hospital and they can’t go around telling people to source their own medication as I’m sure the number of deaths and ailments from miscalculated doses would be absolutely monumental, but there’s got to be some middle ground.

So I don’t have any specific advice, but hopefully it provides some hope that there are some vets that truly want to help and not just for the money.
Hope your darling pooch gets better soon.