When I try with offset the inner curve isn't a closed curve. (The original is) I tried making an inset of the inner curve for fun and that one has a ton of segments where the points of the curve don't overlap so it creates a bunch of separate curve segments.

I tried extruding the outer curve then doing a boolean diff using the inner curve and that looked OK but my slicer tries to do some weird things with it when it sets up the print - some segments get detached moved into the middle. Not sure what's going on there.