Affinity Designer - windows version - is a wonderful product and I'm trying to use it, but there's some show stoppers. Seems to me with the skills you folks have, you could make this happen; the ability to copy a layered project into another format such at TGA. I build something, with multiple layers then copy merge and past it onto a tga as another layer. PSP has this feature. Not sure if PSD does - not using it. I know your export has a good list but TGA is not one of them.
I build a project, then select save copy as TGA. i then pull that TGA onto my PSP work area. I open my layered project, do a copy merge and then paste it over my TGA, The new TGA is then converted to a DDS using DXTBMP, and copied into my flight sims texture folder.
I want to build with Affinity Designer but can't use this for something I do quite often, and I'm sure others would appreciate this feature, at least I hope so.