Endeavouros

type 3 letters to initiate an update you can do this on any distro, just create an alias for the distro's corresponding update command

To answer your actual question, Pop!_OS could probably be a decent alternative if you want something other than arch

to be honest the next step from endeavouros would be straight up installing bare arch, worked for me just fine (if you're experienced enough), other than that, there aren't much options, i think endeavour is as best as it gets in terms of arch-based distros

Any system can match those, once it's configured and you keep it simple.

1. Any notifications can be disabled
2. Use a simple DE like Xfce, or a WM; also, turn off unnecessary services
3. Again, if you don't install much, there's less to cause conflicts
4. Should work with any distro

I like OpenSUSE Tumbleweed. Here's how it aligns with what you're looking for:

1. Notifications depends on the DE you choose; disable the automatic SW update checker and notifications will be minimal
2. Pretty much any DE/WM is supported, pick a low mem one
3. zypper dup every week or two and you're good; make an alias if you're lazy; if something breaks (very rare), boot into the old image and run snapper rollback and try updating again in a few days
4. I haven't used OBS, but you probably need pakman repos for codecs; I doubt it's more effort than opi codecs

Debian stable could also be a good pick, and it's what I generally recommend. Just use FlatPak for most software and the older system packages shouldn't be an issue. It may take more work to get OBS setup though, I haven't tried.