Yeah same, the only solution is for you yourself to post in those niche communities more often, making them more active and thus more attractive for other users.
Welcome to my world! I try to post sports I watch on !sports@beehaw.org as much possible as I can. Same goes for !Football@sopuli.xyz and for !tennis@lemmy.world. Lately is more politics and memes.
It's pretty much a symptom of having a small userbase. The most niche thing that can maintain activity isn't very niche yet. I hope and expect it will improve over time, and in the meanwhile I'd like to see the attitude to Reddit repost bots to soften.