I like injectables like Legends and Adequan as well as plain old glucosamine. Is there any reason why your vet didn’t suggest it?
Here is a little light reading on the injectables.
I will say, out of all the supplements, I’ve seen Phycox EQ do the most for the horse.
You might want to become a fast expert on OsPhos and Tildren, though. They may come in handy to halt and prevent further joint deterioration. (if you haven’t already learned about them).
I’m terribly sorry it is DJD but I would like to add that the lameness vet I assisted was always super positive when it came to joints because bone is a lot easier to treat than tendons and ligaments. So it could always be worse!