Hi there,
Sure, Thing… seems to me it’s a board file issue or overlay but more than likely the board file, after looking more into it. The Zephyr actual board file does not have the same definition as the SDK so there is still an issue with that, either the naming convention is incorrect or they have both the BLE and Sense versions mixed. The Main line repo for Zephyr has it different than the LINK so WTH? Looking through the Boards folder in the Seeed file.
the sysbuild is primarily used when building bootloaders (MCUBoot) and application images together.
no your situation wouldn’t require it.
I would almost bet the EXTernal Crystal stuff isn’t in there.
or if so it’s wack. Debug will get you in 
HTH
GL 
PJ 