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
I recently found pickled radishes at a farmer's market. So good!
You don't need more than one account. You just decide which instance you want an account on, then subscribe to all the topics you care about across multiple instances. I just think that generalist instances with thousands of local topics are unnecessary.
I think part of the issue is that all the different Lemmy and kbin instances are trying to be Reddit themselves. By which I mean there are a bunch of instances with no focus. They're all "kitchen sink" instances, each with their own Politics, Tech, Cats, etc.
Lemmy.world, lemmy.ml, kbin.social, fedia.io. All of them are generic reddit alternatives, but the real reddit alternative is the amalgamation of subscriptions from multiple more focused instances.
Startrek.website is a great example of the opposite: it's an instance focused on one topic, where some people will want to sign up as a user and others will want to just subscribe to one of their three (!) boards from their own instance. They don't need their own Politics topic, users on the site that care about it will subscribe to a politics topic from another instance. The startrek admins and mods only have to care about their one focus.
My ideal fediverse feed would be pulling individual topics from a few dozen more focused instances instead of one generalist instance. I think that's what's going to end up happening.
Careful, he's a hero