Simple Shape masking is far too inflexible to be useful in any situation.
So, I suggest Draw Mask. However, there’s two different iterations of this idea.
Simple Draw (Blue Lines)
Click a few times to indicate lines, create a full shape with no holes (ideally it automatically jumps from the first to last handle) and that’s your mask. Maybe a smoothing slider would be good.
Curved Draw (Orange (and grey) Lines)
This is a different one. You still define lines with points, but you can drag an extra handle we’ll call Curve Handle for the sake of it, and creating a bigger curve in the direction you drag it out. I have no idea what this is called but it’s similar to a specific tool in Inkscape, which if you’re using Shotcut, you probably already have installed.
I suppose you can just keyframe the position of the handles and get the mask from that.
Useful Image to Indicate What I Mean