Want to try adding a btrfs partition but stuck on installer

Snapper seems to be the preferred approach. It should be pretty simple to set up in EndeavourOS with AUR helpers. If you want a more hands on approach with reading up, custom subvolumes layout and such, it can be more complex.

Timeshift is a great and simpler alternative. If you choose that, be sure to take note of this thread, and pebcak’s solution: