Ah, the SIP requirement for yabai isn’t that important IMO. They detail it in their wiki [1]. Incidentally, I tried it out since making the comment, and even without SIP disabled I’ve got it close to where I want it to be. It’s still not as great as my custom i3 setup was, but a few more weekends might get it to a much more comfortable place.
1: https://github.com/koekeishiya/yabai/wiki/Disabling-System-I...
The "Caveat" on the yabai's README page has been enough to deter me away (or rather, make it strictly impossible for me to try it). And it's not even yabai's fault that Apple has made the window manager (WindowServer) mandatory.
So, no usable window manager here so far, yet.