@benoit
Hello, good news, I saw only this morning that you can star-rate raws in Digikam and those stars will be read by RawTherapee.
For that you must use a pre-dev branch of RT, called exiv2. You can find them on this page. See section Downloads.
Once up and running, go to Preferences - Image processing > Metadata sync with xmp sidecars and choose Read-only or Bidirectional.