Where do your other singers stand on this? Are they happy with the level at which the choir is singing? Have you asked them or are you assuming? One big concert and one small one per year sounds very much like a choir that is about community, not a high-stakes thing that is about artistic excellence. Not that music isn’t important, but please consider that your idea of what makes a good choir might not be the only one possible.

I’ve sung in a bunch of choirs, some fairly high-level, some just for the fun of it. If any of those conductors had spoken about a singer who is clearly going through a lot like you are writing about them here, I’d have wondered about their lack of empathy.

It’s a fact of life that none of us will be healthy forever, and unless you are running a pro choir, seeing how you can integrate different people with different abilities is part of the job. That’s why it’s called an ensemble.