I know you're specifically asking about Mastodon but consider checking it firefish as well. It's got a lot of quality of life features that Mastodon still does not. That said the app situation is a lot rougher. Most Mastodon apps do work with firefish but every now and again something seems to break. If you're looking for an instance firefish.lgbt is pretty solid but a little on the small side. Of course everything federates so that should be a huge issue unless you're really into local.

I have an epic experience there

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!

Yes, very active, there is the #introduction tag there where you can find people (and people find you).