Are flatpaks really that bad? Why would they even require more resources?

Sounds like my story. I think I started with Ubuntu, but installed mints cinnamon Desktop environment later on.

I suppose this made you work in it later on as well?

In the top left there is a button to open a sliding menu which comes from the left of the screen. The first entry in this menu will say “Anonymous”. Click it. Now you can log in to a server of your choosing.

Currently you will need to create an account on some instance beforehand. You cannot register from the app. You can only log in.