I have XC-200s I have run them indoors and compared to both a TACX Neo and KICKR Core. Thus far they’ve been what I’d say is close enough considering the differences between them. I have quite a few rides that I uploaded into the DC Analyzer. I do not torque to spec everytime but tend to follow DC’s procedure (I do not have a torque wrench at my alternate training location).
Sorry to hear you are having problems and that Garmin Support is not helpful. I’ve had good experiences with them in the past (U.S.).