I have no idea how you can productively work that way
The reason for getting 4K monitors is not to have less stuff that can make you productive visible
KDE 3.5 was the top of OSS desktops, in my opinion
If I'm switching windows and I'm not alt-tabbing, I usually just hit the super key and select the window from the thumbnails/previews.
I don't find that to be unproductive, but then, I haven't really been even trying to change the basics of my desktop or experimenting much in a while, so it might be that I've just settled for it to some extent.
If I had to first move the mouse to one location in order to get a list of windows, and then again to another location to select the window I want, I don't think that would work at all. But with a very easily accessible keyboard shortcut for the first part it does.
I often have several windows of the same application open at the same time (particularly browser and terminal). When I want to switch from say the IDE to the _right_ browser window, how do I do that in Gnome? (I use Workspace Matrix[0] as I'm a visual type, and that works for me, but perhaps there's a better way?)
Sometimes a more direct approach is to use the action "Switch windows" by assigning it a shortcut Keyboard settings.
I can't answer your other questions. Not only I don't have GNOME 40 but I greatly tweaked Ubuntu 20' GNOME to disable all animations, remove the launcher, move the top bar to the bottom, use those very same horizontal virtual desktops they introduced in 40 (hurrah) and add an old Windows like taskbar. It feels much more productive than the default settings.
I like Gnome default, but KDE does most of what you described by default too.
I kept using GNOME fallback (or flashback?) until 2018 because it worked as I like. I knew there were GNOME shell extensions to suit my needs by then so I switched to GNOME shell with Ubuntu 18.04.
I never researched KDE so I don't know what became of it. A total change of DE is a big thing and could take a lot of time.
I prefer a taskbar at the bottom with one button per window in it, so that I can have an overview of which windows are open, which document each has, and quickly switch to it with mouse (when not using alt+tab to switch between recent windows).
What I do not want is to have to activate something, or have some animation, first, before seeing what I want to navigate to.
However, often we don't need to do that because GNOME encourages us to spread our windows over several workspaces. The animation for switching workspaces is practically instantaneous.