I am impressed you persevered and achieved success with the motors. I hope you document your solution so others might do the same if needed. I bought a spare tool head for my zero after hearing that the model is likely discontinued. I thought it was a good buy considering all of the parts in the tool head. That plus your solution for replacing steppers ought to keep it going for a long time.
I can’t take the credit for the software, I used GitHub - MRX8024/chopper-resonance-tuner: Registers calibration script for TMC drivers · GitHub
It’s an excellent program and managed to really get Kraken V2’s running quite well, The specs it came up with were nothing like I’ve seen from other peoples projects.
I think it’s because if a rsense is changed beyond default, the differences in current peaks really mess with the rise and fall of stepper timing. The program showed that some really crazy settings were needed via hend, tbl and toff
Next time I’ll just replace the resistors on the board and have done with it, I didn’t really want to remove the heat sinks as i suspect they may be hiding somewhere.