Affinity Photo only has entire image/document based cropping.
It needs to have image/layer based cropping.
While one can use selection/deletion (which is destructive) or masking (which can be awkward), I think a simple straight line crop tool (like the document crop) and a bezier and b-spline crop tool with handles/point control would be good. I'd like to see two features in that regard (time to suggest them in the Feature Request forum):
• There should be a Non Destructive Crop
• It should have handles/points (much as the crop tool does). That can work for both "straight lines" or on bezier and b-spline curves. * Ideally it should have x, y coordinate point readout which might help in some cases.