NOTATIO

I've never used Affinity so I'm not really the best to comment on this. However, just for information, I'm using FrameMaker when writing complex (regarding layout, pictures, plots, graphics, dynamic links to embed data from databases, spreadsheets, other documents, etc) and in particular for large documents, the largest so far was something like 9000 pages (a user manual for a satellite). With the latest version of FrameMaker I can still edit documents I wrote 30 years ago.

As a retiree FM will probably be too expensive to maintain. For private use, the free Scribus could be an alternative for me.

My wife used FrameMaker for her work and loved it. Affinity Publisher seems to be fine for what I do, but apparently not for imported documents, which is a shame because other aspects of the program are excellent, although I do see occasional display weirdnesses that clear up with a little time or an undo or two. I think I tried Scribus, but don't remember feeling that it was a replacement for InDesign. Affinity Publisher does.

Loving the open-source projects and software, I have also tried Scribus but it was painful.
Since Adobe has created that monster-sized suite of apps, I have moved all my documents to Affinity Publisher (thanks to @benwiggy mentioning a few years ago) and I am so happy whit it.