Hi! I’m in a similar place - trying to find interesting active newsgroups that aren’t completely overrun with spam. Here are a few that seem to still be going strong:
alt.fan.usenet alt.folklore.computers rec.arts.sf.written rec.arts.tv rec.food.cooking alt.home.repair alt.woodworking
One thing I stumbled upon that was super helpful was my news provider’s daily Innreport, which includes a listing of request counts by newsgroup (example here: http://www.eternal-september.org/stats/news-notice.2023.06.21-04.00.02.html#nnrpd_group). Going through those lists helped me find other active groups for more niche interests than the ones listed above.
Good luck!
For what it’s worth, I’m not on Reddit and didn’t know that this community had a counterpart over there. Happy to have found it on Lemmy, though