You might have configured something that broke it because there ain't no way what you are saying is not supported on Linux.

I know Arch is a rolling release so it doesn’t have that on purpose, but it’s not much better with Ubuntu - I was getting updates every couple of days, once a week at best.

You don't have to update if you don't want to and you can schedule your updates as well with a bash script (although I prefer to do it manually once a week). I have a Windows VM used for MS office and Adobe that hasn't been updated for months.

Window tiling doesn’t exist “out of the box”, you need third party software

It is out of the box. Meta + Arrow Keys and/OR Meta + PgUp. I use it all the time lol since KDE Plasma 5 and Gnome whatever version it was 3 years ago.

Saving window positions (on Wayland) is the most confusing one

Confirmed works by FarrellPerks@feddit.uk in above comments. Although I never tested or cared for it.

SDDM displays the same interface on each monitor, and each is a separate instance of SDDM

I don't know about desktop towers, for laptop it is always only one instance — my laptop display, monitor is dark before I hit enter. And for the normal KDE lockscreen, it does give it on both the screens but I can enter my password in any one of them and logon.

if you disconnect an extra screen, all the content gets dropped on the main screen. Since Windows 11, if you then re-connect the screen, all windows will pop back into their places before the disconnect happened.

same happens on KDE Plasma.