PakaHuszar: Hi-Fi Rack - Motorized turntable tray 3.

This year I decided, to complete some previously abandoned projects of mine. I'd like to finish one of those in every month. It is just a plan and not a promise (I don't want to fell into the trap of unkept new year's resolutions).

The first one in the row (for January) is my motorized turntable tray. The previous posts about it can be seen here:

Since I restarted January, many things happened with it:

Added the final electronics
Changed the TI DRV8825 stepper driver to Trinamic TMC2209
Almost completely rewritten the code, based on TMCDriver and AccelStepper, while keeping the original touch sensor library.
There was an achievement from my side, that after a long struggling period, my 3D printer is back online and working stable, so I was able to add 3D printed parts. This way a new cable management arm come into picture instead of the unreliable tape used previously. It is used for the touch sensor cable and the mains cable. For the audio cable I may create a second arm sometime.
I also 3D printed the holder for the connector at the back panel.
Created a kind of power breakout box (temporary solution until the proper supply is made):
It has:

+12V - Wall Wart adapter for a time being
+5V - Wall Wart adapter for a time being
Earthing (for the touch sensor)
Serial connection

Mounted it into it's final place

While I was writing the serial communication part of the code (it will be used later), a weird issue raised.

The connection to the PC just created unusable data stream. A complete mess. Even I changed the USB-Serial adapter. I didn't understood why. Then an idea pop into my mind, and...

Yes, I used a 12MHz crystal instead of 16MHz. This means, everything working correctly, except the serial communication. When I used 115200 on the arduino side and 86400 on the terminal side, the communication eventually started to work normally.

I didn't wanted to keep it this way, so changed: