Spaces is still there, and it works largely the way it always has. It's called "Mission Control" now.
IMO Stage Manager does provide some useful window management if you spend a bit of time using it, but it's still buggy and missing some key features (there's no way to open a new application and have it automatically added to the group of windows you're currently viewing, for instance).
I used to be an avid user of Spaces but that too can entail lots of manually dragging things over to the desktop space you want.
Because it does a different thing than the Dock. It parks groups of windows. But again, if you don't like Stage Manager, it's easy to shut it off, and you can even turn it off and on from the Control Center.
Totally agree overall, though. There's just no super clean way to organize windows.
Lately I've fallen back on just opening apps and using ⌘-⌥-H to hide all the others behind it. Feels primitive but also "just works". ¯\_(ツ)_/¯