Pastel

Unfortunately not, systemd is tightly integrated into NixOS and an option for alternative backends would have non-insignificant overhead so they aren’t going to introduce it. The next best thing is SixOS

The specific os doesn’t really matter, just get one that runs a lightweight WM. Whatever you pick, make sure you enable Zswap on it to really squeeze out every bit of performance out of that ram.

I like that wallpaper, do you have a source for it?