TMC2209: no UART communication

[tcurdt]

I am a struggling to establish UART communication.

I verified that the RP2040 serial works (by connecting a FTDI).
I then setup the connection to the stepper like this:

R6 is 650 Ohm (didn't have 1k around)
MS1 and MS2 are pulled to GND.

Motor voltage is 12V.
MCU and stepper voltage is 3.3V.
IIUC the motor voltage is needed for the communication to work.

Is the setup correct?
What's next? Check TX/RX with the oscilloscope?
Any pointers?