Give it some time, and I think organically 1-2 most popular communities will emerge for each specific topic and people will then just subscribe to those ones.

This is kind of what I was thinking, too. There's no limit to how many duplicate subs there can be on Reddit but that didn't stop people from eventually finding the "main" subs. Lemmy just needs a critical mass of users first, so that the clear winners are easily seen. With numbers being so low right now, there's no clear winner among duplicates.

Notably, you can follow hashtags in Mastodon, in addition to people. My Mastodon feed is pretty well tuned to my interests because I went through and followed a bunch of hashtags I'm interested in.

However, I much prefer the format of reddit so Lemmy is probably a better fit for me.

Mine said "incorrect login" until my application was accepted. Could this be your issue as well?