Scwm is an old but interesting X window manager using Scheme: https://en.wikipedia.org/wiki/Scwm
Rwind is a more recent related project, in Racket (a big Scheme variant), and probably needs a developer to build higher-level features: https://pkgs.racket-lang.org/package/rwind
Although it's in Haskell rather than a Scheme, Xmonad has been solid as a daily driver, and presumably is worth looking at for a relatively modern WM implementation (compared to trying to figure out the whats and whys of how Fvwm works): https://en.wikipedia.org/wiki/Xmonad
Also the popular tiling window manager i3, and how its Wayland drop-in, Sway, work: https://en.wikipedia.org/wiki/I3_(window_manager)
If you make a Guile-based WM that does a good job of smartphone-like one-app-displayed-at-a-time window management (like the old Matchbox WM did), with status bar and popup keyboard, then I would like to use it on GNU/Linux handhelds, and might be tempted to switch the entire project from Racket to Guile: https://www.neilvandyke.org/postmarketos/