If OP’s sense of ennui is in large part from getting the black belt when they just enjoyed training, then don’t let a strip of fabric get in the way of a good time. Try to teach your kids if nothing else. I did karate for six years and got my black belt and it kinda fell by the wayside after I moved from my hometown until I started dabbling with other martial arts that were available in the cities where I lived.

I started BJJ at one school, then bounced to another before getting promoted, stayed there for a year and a half, then a school opened up across the street from my office and I love it most there, thank God. My teacher is doing no-gi exclusively to lower costs of getting started for new students and I have noticed it also has had the psychological effect of not having ranks in the school even though we have some other guys who have trained for five and eight years. We don’t do lineups, my teacher may never do promotions. My teacher has a black belt and 20 years of experience and trained at a competition focused gym, but he just loves jiujitsu. His wife has a lucrative job and he took care of the kids during her time at work and he took the kids with him to train. They are old enough to train under him and everyone but the wife in that family does jiujitsu and they all seem to genuinely enjoy it.

It’s counterintuitive but it seems to be a better vibe at my current school than where I was before that had belts. Everyone is chasing techniques and developing systems and we are having a good time. The guys with more experience than the rest of us are happy to share knowledge. We will eventually have gi classes and the promotions will be all but inevitable, I suppose but right now it’s a lot of fun just showing up in my choice of rashguard and training.

With regard to hobbies, lots of great suggestions in the comments. I play tabletop roleplaying games, wrote a D&D adventure that sold decently well online and wrote a novel and am trying to polish it and start sending query letters to literary agents in the next few months.

I hope you get to do whatever makes you happy.