What is the maximum length of the TMC drivers signal wires?
10 cm would be fine. That's the short answer, the real answer is that it depends on the UART speed and the exact conditions that you will be extending them. If you expect noise feedback from the motor coils inducing voltages into the UART lines, then perhaps they should be shorter. Keep the high current wires and the signal (UART) in separate places if possible. Also if you can use a shielded cable (e.g. and old shielded USB cable) then you could get away with a meter thereabout.