I only tried one (rill) and it also required a separate config file before I could start spawning windows. Anyways I think this river project is mainly interesting for two reasons:

• Allowing single developers to actually make their own wayland window manager. Possible without it, but not necessarily feasible.
• Making it possible to run a functional wayland desktop under memory constraints. River seems to use something like 100-150MB on my system.

The second point appeals to me, because my daily driver laptop is nearly 15 years old, so every MB of memory I can save is of tangible value. I’ve gone back to X11 on it because of video driver issues on wayland, but I keep an eye out for projects like these.

I just tried it for a bit, and you basically have to create an executable bash script at $XDG_CONFIG_HOME/river/init that launches your selected window manager ($XDG_CONFIG_HOME is usually ~/.config). It will not work unless marked executable, and you do that by running chmod +x on it.

I really like Mocu.

there’s a lot to unpack in that headline

They are just the first small and standalone window managers for X11 and Wayland respectively that came to mind for me. Not necessarily relevant

You mentioned trying Plasma on X11 and Wayland. Have you tried a different desktop environment? Just to rule out Plasma. You could temporarily install something small like dwm or Sway just for testing.