@jkandasa Direct using short (60mm) cables, tx/rx from pro mini to tx/rx on rpi.
GW is using a 3.3v 8MHz pro mini to eliminate the need for level shifter. 38400 baud rate (have tried 19200 and 9600 as well in the past).
As I said before please do not use your time on this. Lets see how it goes with the 'new' mycontroller when it is released and if it is still a problem at that point then maybe it's worth a look.
I guess not many people are running their set up like mine.