Wow, lucky you! What a great setup!
Years ago, before I married and moved away, my dad and I were making plans for his farmette-- I only had 1 horse, and buying more horses was out of the question. My horse would need companion animals but my dad (wisely) didn’t want the liability of boarders riding around his property breaking their necks. We agreed that boarding retirees as companion animals was a good solution. (In the end, I decided not to bring my horse home because my dad LOVED his barbed wire fencing, and that was a hard no for me.) Since your place is already set up as a rehab situation, retired horses might be a nice compromise for your horse needing companions and you needing privacy.
Or, since this is COTH, just go buy some more more horses