Now that I've tried it a few times, both techniques almost work. The issue is that alpha-channel pixels still persist, while I would like all alpha channel pixels to either be removed or turned fully opaque and I haven't yet found a good way of doing so in Affinity Photo. I've found the 'solution', though, it's at the bottom.
In your example as well, @telemax, most transparent pixels are removed, but not all.
There is no denying that the latter is cleaner, less alpha pixels, but there's still alpha pixels for some reason and I cannot figure out why. Here is zoomed in:
The solution to this, I found, is to just use the fill tool on the pixel layer with the mask layer being nested.
If anyone has any simpler, better or cleaner solution, I'm all ears!