I finally got the nRF52840 DK, received the Trackpad dev kits and modified my MPU0250 breakout board so it fits with the nRF52DK. 

Yes, it looks like ... a dev board ! But at least we can get started quickly and start validating the components, test them, try the algorithms, and start developing the software. So far I had good results with the Trackpad, the Bluetooth communication, and the IMU starts giving results but the hardest remains its calibration and saving the calibration parameters over a reboot... More headaches ahead...