Reddit had so many users that hyper specialisation was possible.
There was a subreddit for every tv show. And people are coming here expecting the same. I think we’ll see similar content but a bit less niche.
For instance instead of a community for every tv show it might be better to just post about any tv show in /c/television
I used to follow /r/pizza but I’ve seen tons of home made pizza photos in /c/foodporn
For most niche topics you can probably think of a more general community which will help drive content and critical mass.
Only way to grow these communities with niche topics is to interact with them yourself. If you want c/pizza to grow, post content and interact in it!
Using broader top categories is great advice.
I’ll add that I’m using lemmit.online for some of my really, really niche subreddits where there were only a handful of active posters from all of Reddit’s users. It just mirrors posts from those subreddits into Lemmy communities, and does not sync comments.
For posts which are links to articles or videos which I want to be aware of but don’t need to discuss, that’s enough for me.
For posts which are topics I might want to discuss, I can either repost them to the broader Lemmy community I joined or (if the topic is too specific) know I’ll have to open “that other site” if I feel the need to talk about it.
Note that some Lemmy instances have blocked lemmit.online because they feel mirroring is spammy (especially for non-niche subreddits where a Lemmy alternative is available) so YMMV.