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)
If you want to put a controller on the underside of a keyboard, like the promicro, you can do it with just a standard MX or choc footprint by straddling the switch footprint. But if you swap the switch footprint with a kailh hotswap socket footprint the pads overlap with the promicro's through holes. So to keep the promicro under the switches you'd need to rotate it or remove some of the pins from the footprint, giving you less IO. This is designed to be able to be placed over the hotswap footprint.