I wanted more control over the corners of the shapes I use without having to rely on the Xara Smart Shapes, like OCC > Components > Print & Web Components > Shapes > Shape 7.

The method is simple to apply but does require a bit of thought.
If you place one shape over another, select both and press O, the top shape will punch through the bottom shape.
If the top shape is all Black, you get a totally transparent cut-out.
If all White, the cut-out is still there, just not visible.
In-between is a level of transparency from opaque to clear.

Extending the approach.
Create a curved corner square and four smaller ones centred over the larger's corners.
Rotate these 45 degrees. This make it quick to select angled or circular cut-outs.
Name each in the Page and Layer gallery, if you care to.
Select all the shapes and press O or pick Arrange > Apply opacity mask.

In the Page & Layer Gallery, open the Opacity mask group and select each corner mask in turn and rotate, it change the curvature or scale in place by Shift-dragging a corner.
If you change a mask Fill to White it disappears and the underlying shape can have its corners adjusted as normal.

Adjustable Cornering Toolkit.xar

The design file shows the steps taken to work up a variable corner design.

Acorn

P.S. I added edging and as you can see, it is removed by the masks.
I leave how you get a complete stroke to another day.
You suggestions will be of assistance.