@ticktock a slightly quicker option than editing the slicer config would be to create the two macros (say end!.g and endB.g) and then call those from inside end.g:
;end.g commend out the end macro you do not want to use:
M98 P"0:/macros/endA.g"
;M98 P"0:/macros/endB.g"
OK. Going to 1.21 solved my problems. The G10 command worked fine once I updated (same commands would not work on 1.20). For my machine with a 200x200mm bed the following now work to mirror:
M579 X-1
G10 L2 P1 X200
Well, no luck with the repeater yet. I didn't do a whole lot of debug - just went through the quick start routine. It actually made things worse. The Duet couldn't connect at all if the WR810N was nearby. I can try setting it up as a router instead since I have ports available but at this point I think I will try the board swap.
From the research I have done, it seems that 12bit DACs will not work, and you need to use 16bit DACs. Someone in the Marlin development has already started created code to drive Galvos: