So the edge needs to be the same thickness (9mm) and straight on the same plane -which is the issue. When I do OffsetSrf, the edge is always 90 degrees from the surface edge direction which keeps changing direction with the flow of the geometry. The edge needs to be flat.
Hi Aiden - hard to say without a file but I think I would offset with Solid=No, then ExtendSrf some small distance past the plane, if it is planar, where you want to end the surface, then trim it back. If the surface, as it appears, comes close to parallel with the plane, you may have some unexpected results.