You don’t have to sign up on multiple sites, you just subscribe to specific channels you care about from the site you signed up for. For instance, signing up to lemmy.ml’s Politics, Lemmy.world’s Tech, and fedia.io’s Cats.
For instance, here’s a link to !startrek@startrek.website that you can interact with and subscribe to from your Lemmy.ml account: https://lemmy.ml/c/startrek@startrek.website
Careful, he’s a hero