Another "secret joy" is getting in early to new networks before the brands come rent seeking. Telegram communities have this feel, though many are invite only. Mastodon may be the next big one, and has a bit of an early Tumblr vibe.
However, you can filter instances, and/or sign up on an instance that blocks switter users.
There is activity, especially if you sign up on a huge instance like mastodon.social (although I don’t recommend this). instances.social lists all active instances that you can peruse through.
I'm not sure what you mean about not being listed on lists of instances. If you mean not federating your own instance, well, that goes against the entire premise of the fediverse / federation. Running your instance means it will propagate. Your posts will show up in the instances of whoever is following you.