Manna Harbour

Great!

I meant the default keymap from the keyboard's own config repo, but building from unmodified Miryoku ZMK main might be worth trying too if you can't find anything else. Good luck!

Nice! I assume the 10u version uses the same schematic as the 12u, just dropping the outer pinkie columns? In that case you can use the 12u firmware unmodified.

You could test with the default keymap, check your soldering for shorts, and try USB logging.