Everything is soldered and I'm testing now the hardware before porting William's code to the new FRAM etc. There was a problem with KiCad, I didn't do a DRC, and so all keys were connected only to one pin. But I could fix it with some blue wires:

It works now. Here is a test with demonstrating the 8 possible brightness levels of the TM1638, which so far is not used in the current firmware: