After a long while (my 3D printer waited on parts), resumed the project.

The assembled PCB is out of the storage box:

Note 6 optical buttons in the middle:

The buttons have inverted force profile: the initial force is 35 gram, then decreases down to 11 gf hold force. The full key travel is 2.2 mm, and the button press is registered at 1.2-1.3 mm.  Buttons work with a pleasant metal click; at this point no intention to make it quiet (let people admire a mechanical keyboard on the smartphone!)

The keyboard top now looks like this:

Note the left handle having a spring-loaded latch, which allows the phone insertion and removal. The handle files STL is uploaded.