I really miss the ability to swap out KWin for a tiling window manager. I'm currently using Krohnkite and it's OK, much better than nothing, but after having used a real tiling window manager the difference is just too jarring. I physically need a desktop which is usable as much as possible both with the mouse and with the keyboard so I have to switch as rarely between the two as possible. Plasma on X11 with a tiling window manger was the perfect combo.
The solution would be either for Plasma to do something like River did [1] and separate compositing from window management, or for Plasma to make it possible to use Plasma widgets in other compositors. As it stands now I either have to make do with Krohnkite or go down the ricing rabbit hole with with River and Quickshell.
Have you tried LXQt? It made things easy for me when Plasma 6 broke a KWin script I was using to improve the multihead experience.