I've written some code here: https://gitlab.com/ben-horner/twiddler_layout [after MS bought github, I moved all my gitlab ] to try optimizing the layout.
The idea is that held buttons don't count, and if you release only one button, and press one button, that it should be as easy a transition as two single key presses (almost). This gives you many more chords available at this simplest level of difficulty (single key equivalent).
I think of moving between two-key chords only changing one key, as "walking" the keyboard. The layout I have posted there achieves this for slightly over 65% of transitions.