After the region union, you could also replicate the Trim with Brep GH component’s behaviour, which computes the intersections between the curve and a brep, shatters the curve using Curve.Split and then selects the piece of curve inside/outside the brep using Brep.IsPointInside. This may avoid dealing with seam problems, for the cost of computing an extrusion.
That is mint @magicteddy ! This was definitely above my pay grade and experience. I’m going to need to go a bit deeper than just Python & RS.
Regards,
Adrian