I got the PCB back from PCBWay through their sponsorship program and it looks great:

In the picture the IMU (bottom right) and the sensor pads (bottom left) are clearly visible.
And of course, big center stage for the CR2032 battery holder.

Fabrication quality is impressive, a quick test showed the MCU readings being accurate. In retrospect I should have use both sides of the MCU pins instead of just one, but I soldered a socket, that should work:

I will use the remainder of this vacation time to bring the project to completion and present it to an audience of coworkers interested in learning about "Arduino" things. What's remaining?

• actually putting it together, trivial
• power code; for the advertising part it's done. I will need to code a way to wake up the MCU when the sensor triggers, still haven't figured out - from consumption perspective - what's a reasonable choice
• BLE API. The MCU is powerful enough to store multiple "profiles". Each user can have the "mileage" stored on the MCU and it would be a matter of providing a "switch-user" write characteristic, which seems... logical. Real time measurements of speed and incline are user independent. So... trivial.
• Display: an app on the phone? a separate device? a more sophisticated app that generates a random scenery based on speed an slope? an even more sophisticated app that can simulate the most famous marathon playfields?

Suggestions?

Stay tuna!