Would it be impactful if someone creates a dummy proof guide, including a list of equivalent communites <-> subreddit?

I think that it would, specially if hosted in some third site (preferably a popular platform) and/or spread across Reddit.

Something important however: we should invite people to the Lemmyverse as a whole, not just this instance. This instance is already overburdened, and even if I’m myself staying here (I like it), everyone sticking to the same instance defeats the purpose of decentralisation.