The stitch-together solution was offered but then indexes and contents become a pain - especially if someone wants to replace the text in another language and publish themselves. That extra nonsense will truly put them off.
If I have all the text written up and then imported into layout, then it should make it the most accessible. I don't like the idea of people having to spend money to make their own version (it sort of defeats the point) but if they are going to then £55 once is much better than £120 a year.
LaTex should be fine, my friend did his monstrous 400-pages-of-images PhD in it but I found it, well, arcane. I also checked out Quarkxpress, which is as old as the hills and didn't give me much confidence that it would work for my big documents.
I've watched a bunch of Affinity tutorials as you recommended and I'm going to go with them. It's really, really, really, REALLY, REEEEALLY similar to InDesign. I mean... it's REALLY similar. VERY similar. You know what I mean? Steal-your-customers similar.
Which is good for competition, Adobe has owned this space for too long.