I made a couple of "Help Design Lemmy" posts in !lemmy@lemmy.ml recently to get feedback and ideas, which was very helpful. I will continue to make such posts to improve join-lemmy.org, and also Lemmy itself.

Had a look at the Piefed signup now, choosing categories like that is a good idea. But the question is how these categories get curated. We have something similar with the instance topics on join-lemmy.org but no one is really helping to maintain them. So for community categories it would probably similar. In 1.0 we will have some improvements for discovery, like multi-communities and a "suggested communities" collection which can be set by local admins.

For the Mastodon recommendation there isnt any good alternative software that I can see. So its probably best to recommend a single Mastodon instance, depending on the target audience.