Have you tried 3D Slicer recently? We've completely reworked segmentation by adding a new Segment Editor, which should be comparable and in a couple of things much better than commercial software (not just in price, flexibility and extensibility but in capabilities as well). Segment editor supports overlapping segments, advanced masking, real-time slice/3D view synchronization, hugely improved Fast GrowCut ("Grow from seeds"), contour interpolation, hollowing, smoothing, cutting, Boolean operations, input volume switching, editing on oblique slices, etc.
You can find a couple of tutorials and demos on our lab's YouTube channel and you can find some step-by-step tutorials here.
If you install SlicerRT extension, then you can load RT structure sets directly into Slicer and edit them using Segment Editor (or compare them, register, transform them, export them to DICOM, compute DVH, etc.). See for example this module for DICOM import, automatic registration of prostate MRI to ultrasound, and DICOM export:
If you have any specific request or recommendation then let us know. There may be many things that are easy to implement and make your work much easier.