Hi. You describe so many issues that it is hard to guess. What’s strange is that these issues only occur with properly conditioned PETG. Maybe basic questions, yet: Are the slicer settings (e.g. cooling fan speed, flow rate, velocity) identical in both printers? Is the filament calibrated (flow and PA)?
I’ve never had (worth mentioning) issues with PETG, using either the Engineering, Smooth or Textured plates. You may try a different plate, but I doubt it is a problem.
Many users complained about problems and found the optimum settings: [SOLVED] Bad quality when printing PETG - #242 by Inevergiveup. You may take a look.
You may check if the X1C mechanics (e.g. belt tension) is OK. It may require some fine-tuning.
If the problem persists, can you print in the X1C a test model (e.g. benchy or other you prefer), sliced using standard settings for PLA and PETG? The results may help to identify or at least discard possibilities.