Congratulations, a huge achievement and an inspiration for me (especially the subject matter.)

One of the major advantages of LaTeX is that it will fill lines automatically, hyphenating where necessary.

Another important part of type design is flowing paragraphs so that the final line contains enough type. One does not want a paragraph to end with a single “me” by itself on the final line, and ideally you want a substantial amount of type there.

Are these parameterised in LaTeX? Did you turn hyphenation off?

I don't see how hyphenation could be left "on" when publishing ebooks. Nearly all the platforms I've used allow the user to control font face and font size. Hyphenation would malfunction under such circumstances.

soft hyphens can be put in https://en.wikipedia.org/wiki/Soft_hyphen to allow the client to hyphenate as appropriate.

TIL. Thanks. But this would require soft hyphens in every multi-syllabic word of the epub, right?

Also this tends to be sort of overkill for what most people want so - as with most tech - gotta evaluate if it's worth the time and effort.