I’m not exactly sure how it works with flatpak versions, but for native Steam+Lutris, you install it with this and Lutris picks it up automatically, as far as I remember. Probably need to allow the flatpack to see the installation directory or put it in Lutris runners altogether instead of Steam directory.
Helix is very similar to Emacs and vim/nvim, but a lot easier to set up. Tried all of them but with Helix it just clicked for me.
How would one go about using that sink?