Here you can find (at least for Android) app lists for Fediverse apps (take also a look at the subcategories): https://android.izzysoft.de/applists/category/named/fediverse?lang=en

My answers from Firefish unfortunately do not arrive here on Lemmy. It would be great if it basically worked from Firefish, now. If it is instance-dependent, I would perhaps change the instance, because Firefish offers many advantages compared to Mastodon.

Here you can search for Commnunitys across (almost) all instances: https://lemmyverse.net/communities This exists: https://lemmy.ml/c/epilepsy But it still seems to look inactive

You may use this tool to search communities: https://lemmyverse.net/communities

There are some curated lists that aim to promote smaller fediverse instances in particular: https://joinfediverse.wiki/Instances https://fedi.garden/about-this-site/ https://fediverse.party/en/portal/servers/ If the criteria fit, you can submit your instance there.