I don’t have any more constructive trust suggestions, other than to caution that one of the most dangerous times in the life of any horse is on the death of the owner. That is a frequent cause of a horse ultimately ending up in rescue - or worse.
You are wise to choose a home that you have verified has space for the horse for life, and is willing to provide that care. That’s a major mistake some people make when they do make a plan, they ‘give’ the horse to someone who is not up to supporting it. It’s important that the next owner values the horse, emotionally and/or financially, to be sure of the future.
Not to scare anyone, but if the owner hasn’t put together a plan … It doesn’t matter how valuable, trained or talented the horse is, horses from low value to very high value have ended up in bad circumstances because the heirs had no clue. Heart horses can end up neglected, or cycling through one auction pen after another, or even on the truck to slaughter. Even a purebred, breeding or show-winning horse that has considerable market value in the right market can end up in dire circumstances. And it can happen quickly, within days or weeks. It is shocking and unfathomable, but it happens every day.
If you do daily feeding and care and your family is not involved with that, be sure to have a day-of and days-after plan that your family understands.
If your horse is boarded, make sure your heirs will know where. Even if they’ve been to the barn before, could they find it on their own? Do they have the barn name, address, phone number, barn manager name?
Several years ago I realized that if I were not directing my own affairs, my family wouldn’t have the first idea how to find my horse. Or even how to get in touch with the board barn. And I didn’t know how they would go about finding that information, given their complete lack of contact with any of my horse connections. Honestly with everything they will be dealing with in those circumstances, I’m still not sure how soon they will think of doing something about my horse (I have one horse-loving niece that I’m counting on!). Fortunately my horse is at a good board barn with a very responsible and caring barn owner … but even that barn owner is unlikely to want a new “free” horse on their monthly bill. That plan is a little bit complicated.