There is a bug in 3.30.1 making it sync everything, in a loop. Keep an eye on your data usage! https://github.com/nextcloud/android/issues/13738

I've only been on mastodon for a couple of days, but yes it is active, and depending on what you're after it might we worth it. For me, it seems to be a nice mix of keeping up to date with things and people I already know, and at the same time stumble upon (related) things I did not yet knew about.

You will have to put in some effort to follow people/accounts/hashtags, but that's kinda fun. Just try it out!