The general feedback I have is to remove all formatting before you start laying everything out and use some simple convention for stuff. Typically you’d use one color for emphasis, another for pull quotes and such, yet another for headings. This way it will be much easier to import it into your editor of choice.
This is something I think I didn’t mention before but: don’t invest in a lot of art unless you know what style you’re aiming for and what general layout you’re going to have. Polish two to four pages from your document in Affinity Designer or some other tool you’re familiar with/have access to. Make sure you include most of the things that will end up in your layouts so, for example, if you plan on having tables, have a table there; if you want vertical art, have it there. It’s crucial that if you plan on sharing it to gather feedback, you should commission the art pieces and shouldn’t use random placeholder art you have no rights to - but I’m sure you’re aware of that.
I can’t guarantee that but I suspect that you won’t be satisfied with your first super polished attempt. But that’s OK since you’ve wasted time on two pages instead of eighty. Even things like the number and sizes of columns, margins and gutters will take time to get right. And these things interact with the desired sizes and proportions of art you can have. You may realize that in order to improve access you need page headers with section names. Size, placement and presentation of page numbers will matter. And so on.
What I’m trying to say is: polish a tiny vertical slice of your book first to get an idea of how much and what sort of art you need before you pay for it (among other things). And try to stick to fonts that are free for commercial use so you don’t end up with a problem on your hands.