It sounds very surprising that uncompressed RAW should give you any problems. Usually, it is the compressed ones that can be tricky.
Which OS/distro do you use? Version?
What version of Rawtherapee?
Can you upload a sample image using https://filebin.net/ and post the resulting link here?
Lens correction is handled via the lensfun library/project. I wouldn’t be surprised if the X-T3 isn’t in there yet, but you can make the measurements yourself to correct your lenses. Or from the terminal run lensfun-update-data (I think!).
Your raf opens just fine in RawTherapee as well as in darktable.
True that rt/lensfun does not recognize X-T3 (yet),
but dt handles profiled noise profile fine.
@Claes yes, it’s Canada and it’s -20 already (today). The RAF opens find in the latest darktable (link provided by @darix ) but there is no lensfun database update for it.