Use a site like browse.feddit.de to find communities you want to join and join them. Every instance only "has" their local communities plus whatever remote communities the users of the instance join. With more users it is more likely someone else has subscribed to something you are interested in, but someone on e.g. lemmy.world had to be the first user there to search and subscribe to any community that isn't based on that instance.

That is how it works you have to manually add communities

I just started running my own instance as well and have similar concerns. Interested in what you wind up doing.

I can vouch for community seeder, my personal instance all page looks as populated as my kbin.social account.

Lemmy Community Seeder works pretty well. You may want to experiment with which servers you pull from though ( beehaw consistently gave me errors which caused LCS to crash). You can set up a specific account without any admin or moderator privileges if you don't feel comfortable using your personal account (all the account really does is subscribe to the top communities from the servers set in your config). As far as I can tell you only really need to run it once in a while to get an updated list of popular servers.

LCS is pretty great but now my database is growing pretty big. To some extent, I guess it's something that will eventually happen anyway. Might as well start early?