Five months in with vetoryl, advice needed

I have a 9 year old boy - boxer/American bulldog mix - who was diagnosed with cushings disease in February. He had the hair loss, pot belly, thick throat, and started having accidents occasionally in the house. Leading up to the hair loss he’d become lethargic and I attributed it to his other condition: polycythemia vera. Once he started losing the hair I got him into the vet immediately to be tested and he’s been treated since. I’ve had him on 30mg once per day of vetoryl. He also takes hydroxyurea for the polycythemia vera five times per week.

Some of his symptoms have changed but not by a lot. New ones have emerged. He’s still lethargic, some of the hair has grown back but not fully, he has developed a dark growth on his tail that he doesn’t like touched, and in the past two weeks has developed a slight limp in his front legs which I assume are due to muscle wasting.

I took him to the vet yesterday to get his cortisol levels tested. The vet gave me some gabapentin and another med that’s an anti inflammatory for his leg. I received the results today from the cushings test and his levels are completely within normal range. He’s in the lower 1/3 of the range, ie not having an addisonian crisis. He wants me to keep him on his the gaba and anti inflammatory for 7-10 days to see if there’s any improvement to the leg.

All of that said, shouldn’t I have seen more improvement by now? His personality is still there when he’s not zonked out. But he’s overall nowhere near the dog he was this time last year.

Everyone says you’ll know when it’s time, and I think it might be. Should I have seen more improvement by now? The vet seems to think he might never grow back all the hair and that given his age his energy won’t be the same, but I’m scared I’m in that trap where I’m letting false hope keep me from doing what’s right for him.

He doesn’t seem to be in outright pain. But I don’t want him to be miserable. It’s been since late December since I last saw him play with a toy and he’s just generally uninterested in any of that. He also used to be reactive to other dogs, and now couldn’t care less when he walks right next to one.

TL;DR: not much improvement after five months. Is it time to set him free from this?