Betta Fish: Chronically ill betta, time to euthanize?

My man was a write off at a store I work at due to swim bladder issues. I’ve had him for 2 years. He spent 1.5 years in a hospital tank (with treatment), never improved, didn’t get worse though. He spends every day lying at the bottom since he has no buoyancy. However, very responsive and still loves to eat.

After cycling a planted tank a few months ago, I put him in hoping the more enriched environment would perk him up and strengthen his swim muscles. I knew there was a risk of his condition worsening due to him being immunocompromised, so it was more of a hospice situation.

While he had a good time, after a month he started losing more colour and his fins started rotting. He also appeared stressed. Planted tank parameters were perfect though. I removed him and put him back in the hospital tank.

My dilemma: He’s very bored and lacks enrichment in the hospital tank, but there’s only so much I can put in to keep it easy to water change and reduce bioload on him. Also, due to the frequent water changes, plants don’t survive in there, and it’s not economical to fully fertilize every second day.

He’s been struggling for years and I don’t think this is a fair life for him, but I can’t tell if we’re at the point of euthanasia. Any suggestions are appreciated ❤️.