IMO you are doing the right thing to go ahead with your decision, OP. You have given this horse a good life, and every possible chance. I totally agree with you to let him go on the best day he has left in his life, not the worst.
I’ve had to do this once and it is desperately hard. But I’m glad he had a great last day, not a terrible one.
Looking healthy is not the same as being healthy. It’s unfortunate that other people choose to challenge these decisions because the horse has been well-cared for and the outer wrapper ‘looks good’. Maybe even everything is working but the one thing that dooms the horse to a future of pain and suffering. And it isn’t apparent just by looking, for some people in any case.
I would not let a horse in this kind of twilight situation go to any other home. It is an open door to horrors. No promise by the next owner will be rock solid, just because life can happen to anyone. Everyone has the best intentions in the moment of the transfer, but then things change down the line.
I have heard so many stories of “we had a stipulation in the sales contract of [first refusal] [when they no longer wanted him, he would come back to us], but the next owner didn’t honor it and moved the horse on, and we lost track of him”. I have heard very few stories of when such a clause was honored and the horse returned to a safe haven.
It’s best to keep him 100% safe from future harm and suffering, just as you are doing. That’s my opinion. Thank you for what you have done, and are doing, for this horse.