I finished the ZMK conversion for my Yushakobo Primer61 60% PCB I got in Tokyo, at least to my satisfaction. My Mac Mini actually recognized the wireless, and while I had some trouble getting a keymap with Bluetooth keys to compile (user error, I’m sure), a ZMK Studio build compiled fine so I just added the bluetooth keys manually. The Primer series are designed to use a locally designed BLE Pro Micro replacement (and their slightly noncompliant QMK build), but I didn’t buy any while there, instead grabbing the NRF52840 Nice!Nano clones I mentioned before. The PCB is meant to accommodate coin cell or AAA batteries, but with the charging circuit built into the MCU, I got a small 300mAh LiPO battery with overcharge protection, bridged the pads for the diodes that would normally prevent charging of the disposable batteries, and wired it into the On/Off switch.
With a Japanese Duplex matrix and no interrupt pins, ZMK can’t sleep like it was designed to do, so I don’t expect battery life to be great, but it works, and if battery life is truly dire it also works fine on USB-C. I am already getting towards the edge of my expertise with this stuff, so I am not inclined to go beyond the capacity limitation for battery health (~500mAh). I also soldered in the TRRS jack, but I’ve read that these N!N clones don’t play nice with TRRS split connections, so I’ll probably let sleeping dogs lie there as well, at least for now.
Now I’ve got to finish the build. I need to install stabs, design and cut a plate, solder in some switches, and then design and print a case to work with the MCU and battery off to the right of the keys.