As nobody else mentioned it there is https://lemmyverse.net/communities to search for communities across instances.
I'm aware how finicky it sometimes is to subscribe to new communities on other instances. The fediverse (Lemmy+PieFed+Mastodon+everything else) has some technical quirks but for me its a small price to pay for an unbuyable platform.
The obvious problem is that niche comms are not active. I feel like this comes from people expecting or even emulating the same experience as on reddit. But before we even create new niche comms. We should use the more broader ones even if the more broader one is casualconversation or something generic like that. Even some half dead forums feel alive cause they have like only 4 different places to post. And is also how reddit started but people are forcing it to be like it is right now.
And to answer your question no not that I'm aware of :)
There is https://lobste.rs/ which seems to be working well mostly because its invite only. Truth be told Lemmy will eventually also have to close registration and have an forced application form. If we want to keep it as human as it is right now. Don't think there is any other way of dealing with bots that are eventually going to hit us.
There is also https://tildes.net/ which is less active but more moderated. They also have only broader comms because its more opinionated.
But they all do not federate which is in my eyes a deal breaker. The mods are probably alright but having the option to move instance while keeping most connections and communities only changing the server owners that control what you see is a game changer for me. It even made me post way more than I ever did on reddit because I feel like its more sustainable. You feel like you are part of the network more than on any other platform, because nobody really owns anything. That is what makes this place so special :)