To close the loop in case anyone else is wondering how to fit a metric (straight thread) probe into an NPT (tapered) plumbing:
Johann contacted me the same day he started the thread here and indicated he'd rather use the metric probe he has so I spent some time on McMaster Carr trying to find an adapter to go from the M14x1.5 metric probe to the ½” female NPT Tee but came up short. They do have some metric -> NPT adapter bushings but nothing in M14 size.
Some of the other adapters I saw were in the $30-40 price range so it would likely not be inexpensive either even if it did exist. The adapter would also have to be long enough to ensure that the probe tip fits inside the Tee adequately which may be hard to confirm ahead of time if complete details/CAD drawing were not provided for everything. Last but not least, Johann's not in the USA so ordering from McMaster Carr or similar places is problematic.
I highly suggested that we simply swap the probe tip (the part that is threaded) for him and that is what was done.
Fitting a square peg in a round hole isn't always easy... sometimes it's easier to just use a different peg.