Yesterday I have discovered a similar firmware project:

https://github.com/I0x0I/DIY-A-BLE-Keyboard
Since it pretty much fit's my needs I have started to adopt it for the keyboard part.

During the bringup I had to realize that I have messed up the PCB (and the whole design) again:

After deeper inspection the YJ-14015 Chinese module  has two unconnected pins:

the P0.26 and 27. Supposedly they are used for the 32.768 kHz oscillator. I have no idea how did I missed it because they have even left out the silkscreen for these pins.

Anyway I will take a look if  I can eliminate the LF osc, and remove and hardwire to those pads. Missing CAPS lock LED is not a big deal but I one of the column outputs is wired to there unfortunately.