If you can live with having some less IO and flash, consider looking at the nice!nano. It's a nRF52840 board in Pro Micro form factor, originally designed for DIY wireless keyboards.
Cheers. While the nice!nano is great it's the additional gpio on the nRF52 I want so I can do 24+ key diode/matrix-less split builds. I might just attempt my own breakout when I get a bit more comfortable with KiCAD.