Congratulations on caring for this little guy for 19 years! I've no advice beyond what you've covered, which is to find a vet who specializes in amphibians. While I don't know what's near you (I'm in another country...), here are some resources that might help you locate a herp vet near you.

http://www.anapsid.org/vets/

https://arav.org/


I will say that every experience I have of bringing a not-cat/dog to a traditional cat/dog vet has been a waste of time and money, so finding someone with reptile/amphibian expertise is an important step.