Well, Affinity has just changed the ‘very expensive’ part. They’re no harder to learn than any other software.
I’ve uploaded entire books as PDF to Amazon KDP. I can’t see republishing being an issue if you have the original native file.
I don’t use Word (or any of its substitutes) and have always used PDF - but I typically don’t do reflowable books. I’m currently on my first attempt at something for Epub and have used Pages after it rendered a fairly good test export. My preferred choice is InDesign but getting the reflow right seemed problematic - especially if you want to also submit to multiple platforms (but I admit I have tested exhaustively).
The problem is that I did not have the original native file. The original book went through five editions, the last in 2006, and for each one new edition they just had me add changes, apparently thinking they would only need to reset the changes. It was about a fast-changing field, so there were lots of changes, and nowhere did I have an original native file. What I did was scan the book and use a PDF image PDFs for the 800 page book. I had to make small changes on a few pages and that was enough to be a pain, but it has earned me a nice profit selling PDF ebooks and print copies of the book.