On the other hand, Firefish have successors in Sharkey and IceShrimp

Kbin has mbin.

Everything feels very day 1.

Because it is.

Lemmy has 2 main devs.

Piefed is developed mostly by a single person.

Mbin has a few more, but not much.

There’s only so much development teams this small can do.

To come back to your question above: you can use https://fedia.io/ to connect to both Lemmy communities and Mastodon.

It uses Mbin.

I think you can import your subscriptions from Lemmy (not 100% sure)