Whatever you decide, you should feel good about giving this horse such a long, quality life.
Your post reminded me of Man-o-War and his groom, Will Harbut (it's a meme that's been circulating on FB lately). Will and Man-O-War were best buds in the later part of their lives > A LIVING FLAME: WILL HARBUT AND MAN O’WAR | THE VAULT: Horse racing past and present
I'm going to go cry now. Good luck.