Check out Louvre. Wayland compositors may not be as hard as you thought.
replyOr build on River as this article suggests.
How is a WM not just a simple plugin/extension? Find a display server you like and write an extension for it!
replyThat would suffice if I were only looking to build a WM, but my goal is a full (lean) DE.
replyThe xserver may not be completely dead either:
replyhttps:/github.com/X11Libre/xserver