Did you try any of the more common image editing software? Both MS Paint and macOS's Preview allow the common workflow of open → edit → save.

Affinity Designer isn't an image editor. Consistent with its name and marketing, it is graphic design software.

Obviously I have no objection to GIMP positioning itself as a graphic design program rather than an image manipulation program. However, if that's the direction they wanna go, perhaps they might consider changing their name to something other than "GNU Image Manipulation Program."