An update on the project a few months later 
First completed proto:
Second almost complete, back side view without the foam:
The first prototypes where manufactured and validated 
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It sports a unibody-split 5x3 + 3 mods keys per side + 3 thumb keys + a nav cluster in the middle.
It has per-key LED matrix as well. It uses a RP2040, and a recent QMK.
Its stack-up is made of a 4-layer 0.8mm PCB glued to a 0.8mm aluminum PCB. There is EVA foam padding to grossly match the FW16 profile underneath the keyboard. Plus a thin sheet of mylar for isolation. It also has the magnetic elements needed to fit properly on the FW16.
That makes its overall thickness (ie. where there are no trough underneath) to basically match the thickness of the One Key Module: 3.65mm 
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- PCB design files
- Fabrication files with ordering guide (NB: a new revision will come soon)
- Firmware source files
- Assembly instructions (will come later once I finish the second one)
Concerning the layout, I know very well in the ergo keyboard community there cannot be one single layout that please everyone ^^.
This one is simply based on my personal one. It does not prevent making future variants with different layouts. Or to make your own.
Because there is another thing this project brings: template PCB to make your own.
You can find it here: keebs/Framework/Kicad templates at main · AxelVoitier/keebs · GitHub
It basically has the PCB layout (for both the main PCB and the alu plate), a fully routed RP2040 and IS31FL3733B for per-key LED, and pre-fanned out tracks for all rows, columns, and their LED variants. It supports by default 5 rows and 13 columns. But there are still some pins available on both the RP2040 and the LED matrix driver if needed.
The electronics is minimally tucked away in one corner. Though it should not be too difficult to move it around in case your layout really need to fit a key there.
And the last thing this project also have is base footprints for both the One Key Module and boards fitting the FW16 (the A1 form-factor only for now). You can find them here: keebs/Framework/Framework.pretty at main · AxelVoitier/keebs · GitHub
I will also put up a soldering guide for the OKM soon.
I would like to thanks particularly Framework for selecting my project, providing enough OKMs for this and future prototypes, reviewing my design, and sponsoring the manufacturing of this first one 
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If you attend their Next Gen event in person today, you should be able to see the keyboard in their developer corner 
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