Hi ykonoclast,
At the risk of drifting OT, I've a couple of comments on your interesting post.
In 2017 I had to move to 1.5.x because I needed to mix French, English and Arabic. Arabic, which is right-left, can't be done with previous versions. The new facilities for text are a huge improvement, and you get used to the ambiguous arrangements relating to where and how to type (in the frame, with separate editor, using styles or not). As you say, there are a few difficulties and the occasional crash, but nothing to worry about; I have never lost anything. Compatibility wasn't a problem for me because I was able to move old stuff to the new version and everything worked.
If you have complicated texts, it's definitely a good idea to move away from wordprocessors which, apart from anything else are not very good at justification and all that (they don't need to be for office work). It would be worth learning LaTeX, which is FOSS but probably the engine used by expensive commercial software. LaTeX is definitely not limited to maths and science. Scribus has all the typesetting facilities a professional would use, but LaTeX does an excellent job practically automatically. There are plenty of tutorials (every university has its guru), though an easy way to start is to take a subscription to the online service Overleaf, which offers a great deal for free. There is a big compilation of Templates including books of various genres. Overleaf is designed for collaborative work, and two people can edit the same document simultaneously online with the free version, which is good fun. I also use LaTeX on my computer and much prefer the TeXstudio editor front-end to Overleaf's offering (there are several available).
It's true that LaTeX has a learning curve, particularly when you want to carefully juxtapose text and figures. When that gets complicated I move to Scribus. Scribus does 'render frames', whereby you can insert rectangular frames of LaTeX text and adjust them to suit your layout - it's a partial and limited solution to getting professional print quality without being a typesetter. I'm writing this mainly because the render frame system lacks documentation at beginner's tutorial level (the same applies with render frames in Inkscape). I've been using LaTeX for ages but still can't get my head around the way to incorporate your own preamble code (fonts, spacings and so on). Perhaps a few templates would be sufficient to get started. Unfortunately I'm getting a bit too old to work on it myself.
Finally, I've written elsewhere about the need for a better international framework for supporting FOSS software.