So I took a chance - what's the point of experimenting if you aren't willing to risk a 50 quid piece of kit?!
It works as I expected, I had a look through the TMC2209 reference material and so long as I'm happy with the defaults I can run it. If I wanted to chance things I can potentially wire the UTXD1 (ESP comms) to the UARTB pin of the Mini 2+ - as I am using the ethernet board that doesn't use these pins. Using M118 P5 S"xxxx" GCode I could send the desired values to the appropriate registers - however I would need to calculate the CRC values for the TMC2209 manually (doable, as the algorithm is noted).
Just thought I'd update on the success in case someone happens to hit this thread with similar issues or questions.
Happy New Year all.