I tested it a bit in a VM to get familiar with pacman and yay. Latest KDE Plasma 6 and more snaps in Ubuntu's future are the main reasons I want to switch.
As I don't use a separate home partition, I have an extra drive with BackInTime home dir backups and virtnbdbackup snapshots.
Is EndeavourOS stable enough for everyday use and would restoring home with BackInTime just work (as root user)?
Nix has been around for over 20 years, and a lot of nerds volunteered to make it better.
Nixos not being fhs compliant is a most likely also a big factor. Before you could emulate FHS you couldn't use language specific PMs like npm, and instead were forced to build those packages with nix. So now it's filled with packages that other distros wouldn't ever add. Even worse, I've seen python libraries have separate packages for different python minor versions.
The Arch USER Repository is is essentially only used for packages that aren't in the official repos. It's not a good comparison, as nixpkgs is the official one.
Depends how you count it. Debian has between 120-200k apt packages pre-compiled for it on the various branches, making it the absolute largest overall in terms of readily-available packages.
If you count by source packages then it's either Arch's AUR with 85k or NixOS with 80k. But I'm not sure about these figures because both these repos have a low bar of entry and not all packages in there will work.