Have you tried teaching anyone else piano or music theory?
I certainly wouldn't recommend starting with a typical book, but have you thought of writing up something like a blog or a small printable PDF of information you would have found useful when you started playing piano?
Have you thought about finding small venues where you might be able to use your skills to teach others, bring joy, or just fill a void for others?
Heck, my mother took a super part-time gig to be the organist at a single family church, just so that she could play for other people (apparently, our family didn't count, but we also didn't have a piano or anything). Literally, before my mother "joined" them, it was a small mobile office structure, with a pulpit, electronic organ/piano, a few well-used pews from other churches, and a few decorations and the only members had been the dad (who served as minister), the mom, and a daughter. When I say "small" I mean it in every sense.
Our mom actually got a lot of joy out of it, the family absolutely adored her, and they were so happy to have someone playing the organ for them. (It's honestly one of the strangest memories that I have of my mom, but it meant so much to all those directly involved, that it is undoubtedly a wonderful memory for all of them.)
Depending on where you are are, I suspect that there may me senior centers, or small playhouses, that would be so happy to have you tickle the ivories.
Just sharing what you know, in whatever way you can, can bring you a sense of accomplishment, and can bring others knowledge, companionship, joy, comfort, etc!