The logic to combine curve depends on the input.
Here graft has a limitation because there is no merge if 2 curves are too close.
The good way could be
Offset the whole set of curves with Clipper without graft
For each offset output search curves that are inside (the holes). Use that to make the “border surfaces”. There was some discussion on that king of problem on this forum, I think it was related to fast way to draw buildings using GIS building contour.
Yes high resolution means too many controls points. For some reason Boundary surface doesn’t like it, it is more a limitation or a bug of the tool.