Since Slicer license allows free reuse for any purpose without restrictions, you can extract any part of SlicerRT DICOM importers. Of course then it is your responsibility to maintain the code and redevelop all the other features that you will need in the future.
Yes, this can be a tough decision. Often by the time it turns out that building a clinical application from scratch takes more effort than anticipated, already so much time is invested into the development that is hard to make architectural changes - even if it saves a lot of time in the mid/long term. If we can help with any information or other help to make the decision easier then let us know.