This isn't great software, but it does seem to work, and it was definitely quicker (or at least slightly more fun) than trying to alter QMK again.
This is an implementation of a "40%" layout -- 24 keys per hand -- with extra "layers" to bring the symbols/numbers and/or navigation keys onto the positions of the normal letter keys.
Here is a photo of the particular keyboard design I'm talking about, just so you get the idea.
See also: same thing in Rust
See my interactive illustration for a brief explanation of the 40% keymap I use.