Is the Vref trimmer relevant on TMC2208 in UART mode?
The potmeter does not work in UART operation, see e.g. this quote:

You don't have to fiddle with jumpers to set your micro-steps, just
change it in the firmware, and you can dynamically change the amount
of current going to each stepper motor (no more adjusting that small
potentiometer on the driver board) just by sending a GCode command
(M906).