Framework Missing Keys

I realised that Framework have provided a KiCad PCB footprint for the one-key module, but I don't think there's a matching schematic symbol, so i created one. Of course, now I'll discover that there was one all along and I'd just missed it...

I've started adapting Framework's "Microcontroller Input Module" reference design to try out potential PCB layouts. Because I want the keys near the bottom of the module as much as possible, next to the arrow keys, I've moved the microcontroller to the top. And I've staggered the keys left and right instead of making a vertical column of them, partly for ease of recognising them by touch, and partly so that I can get PCB tracks between them. There's not much space because the one-key-modules mount to the bottom of the PCB so there needs to be a huge cutout for the keycap to poke through to the top. So I need to make sure I've left enough space for that. It's complicated more by the locating lugs which also eat into space around the modules. This is what I've got so far:

I'm not sure yet if it might be possible to fit in a fifth key. It depends how much the space "below the line" with the contacts needs to be left clear, and whether I can squeeze the MCU in one of the gaps next to the keys (with all its tracks and supporting components...)

Well, Framework are determined to make my life easier for me. Turns out that:

a) It's possible to make an input module as a single PCB - I was thinking I'd need some combination of overlapping PCBs and/or some 3D-printed parts. Maybe that will yield a better aesthetic result in future, but as a first cut, that's ideal.

b) The "magnetic attachment" is a solved problem - there are solderable threaded inserts which are ferromagnetic, and will stick to the magnets in the laptop. Framework have even provided a part number: Keystone 24929, and they're available from Mouser.

c) Framework have produced a reference design, in Kicad (which is my preferred PCB CAD tool), with an RP2040 included. So I can use this as a starting point for my design.

d) Someone has already created a Kicad PCB footprint for the one-key module

This will give me a great head-start. Next step: modify the reference PCB.

So, I've got mechanical hardware that will fit: The framework one-key modules are designed to mount to a PCB, and I can design and fabricate PCBs. So I next gave some thought to the electrical planning.

I know the Framework keyboard uses an RP2040 microcontroller - from the Raspberry Pi Pico. I've never used those (my standard choice is STM32 chips) but I figure it makes sense to use the RP2040 for consistency with the other Framework hardware.

The electrical connection to the input modules is USB via a pogo-pin connector and they're held in place by magnets:

The pogo pins provide +5V power and ground, USB and a board ID resistor:

I can create the connector as pads on a PCB, so I'll need to find a way of making the PCB magnetic so it holds in place.