After my late night rant post above, woke up this morning to find a bunch of responses from Affinity users and the developer. Without having yet gone back to delve into them, it appears that Affinity 1.9.1 now has an "Astrophotography stack" function that handles FITS, including debayering, so it looks like they've started down the righteous path
My apologies to them.
Affinity has a free trial period, it appears to be out of the UK, and there are numerous Youtube videos on using it for astro final image processing purposes. It's worth a look.