Except for the fact that with Lemmy it is still impossible to mention a user in the first post of a thread, Lemmy is an excellent product, architecturally stable, ergonomically well-made and very easy to use.

Mbin (Kbin is dead!) is a very interesting project and perhaps in the future it could surpass Lemmy, but today it has an interface that is still too confusing to be usable. Piefeed, on the other hand, seems to have been developed to reproduce many of Reddit’s toxic dynamics, but the state of the project is so little advanced that any comment would be risky.

To answer your question, therefore, there is no alternative to Lemmy today to improve the user experience.

If instead you want to experience the Lemmyverse (or the Threadverse) by correctly viewing the communities (= the Activitypub groups) without giving up the social experience, I recommend you try Friendica, the most powerful and versatile software in the Fediverse, even if its interface is a bit vintage and the only app is objectively inadequate.