Having always been somewhat annoyed I can't simply place a Pro Micro (or for that matter any other controller) over on top of a hot-swap socket without either turning it sideways, loosing pins, or some other creative solution.
So after trying to find other smaller controllers, but always ending up with a compromise, I finally got fed up with it, and started designing one of my own that is just large enough to fit the purpose.
RP2040 powered of course, and with a mid mount USB type C, the design is extremely low profile and fairly barebones with no status LED, no buttons, etc. making it easy and cheap to produce. And with 26 pin, there are 23 IO pins available for matrix and other things. VBUS detection for easy use with split keyboards, but beyond that stripped of anything fancy.
The boot/reset signals are available as castellated connections next to the USB, and only really meant for the first flashing/emergency flashing, as the rest would be handled by tapping a keycode to enter bootloader from within QMK/ZMK.
Edit: Added D+/- as jumpered breakouts on pins, se below. Also added a pin high/low for assigning sides on a split (useful for handwiring)
I guess an image says more than a thousand words?