That’s one thing I don’t think has been fully figured out yet on Lemmy. (Communities on separate servers aggregating similar topics). My current solution is to try to crosspost between my home server and the largest server with that topic where applicable.

My idea for Lemmy is possibly to assign tags at the community level (or maybe post level), so that a user can look at combined page that puts together all communities on federated servers with that tag. This could function similar to a Mastodon hashtag, and any smaller niche community that want to appear on more general servers (e.g. a Minecraft community could also want to be on #gaming. So beehaw.org/c/#gaming would combine gaming@beehaw.org, gaming@lemmy.ml, ttrpg communities and perhaps mastodon posts with #gaming tags. #tech could include privacy, foss, and similar. Kind of an idea spitball but I think it will help solve the fragmentation problem while still allowing community managers to decide which broader categories they associate with.