I board retirees so I have faced many end of life decisions. I am fine with putting a horse with issues down!! But my experience is informed by 2 occasions where we were within minutes of euthanizing and I decided to wait just a little longer. In both cases, the animals (1 horse, 1 donkey) lived happy, healthy years after I came so close to putting them down. So I am cautious. If it’s a matter of shortening their life by a few months, no big deal. But years are another thing.
OP I would be guided by your vet who knows your horse in real life. I don’t mean my post above to mean you should not euthanize; I cannot assess all your horse’s issues over the internet, but your vet certainly can. You sound like a very caring and responsible owner.