I’m guessing what happened is that EndeavourOS pushed out the update to KDE plasma 6, and if you have a custom theme installed in SDDM, it may not load and display a weird image with an error message. The fix should be to delete that theme, and install the version updated for KDE 6, but not all of the themes work yet. The default theme works normally.
Nobara is a Fedora variant that includes tweaks for high-end gaming systems to get the most performance possible. It is not an immutable distro.
I’m afraid I cannot answer your remaining questions, however, but Nobara has an active Discord community, and you can also check their Wiki for answers.