You DON'T want to spend writing time doing any formatting and playing with computers ... that's an easy was to burn lots of time. Get the ideas (words & pics) clear and complete, then the formatting is quick.
And whatever you use, it should do everything with styles, not manually selecting bold-for-this and sans-for-that - that's just a waste of time (so avoid MS).
Indeed, many publishers are set up to accept manuscripts in Word documents as the input to their editorial and production workflows, which generally use other software for the actual typesetting and e-book production.
Bottomline: use the simplest tools you can and don't let things (or classes!) distract you.
You can use Pandoc to generate PDF's then send out pdfs via Google Drive so people can comment and add suggestions. This was really useful for proofing, but having to pull up two screens and make changes to source instead of the annotated version was a bit painful.
Word .DOCs are sluggish to navigate when they get long, tempting to get sidetracked when you're supposed to be fixing a specific chapter, and don't provide a convenient place to stash meta-data (notes, outlines, scrap, alternate drafts, etc.)
The author Julian Smart is also the creator of WxWidgets and Jutoh is very fast at compiling Ebooks. He is also very good at support; I suggested a feature and he had it implemented in a couple of days. I created a 350 page non-fiction ebook.
I also include links at the end for other flows like Jupyter Book, Asciidoctor, etc. Markdeep  might suit you better for a simple solution that allows diagrams, equations, etc.
Start with Stephen King's On Writing: A Memoir of the Craft.
Then read more. Fiction, nonfiction, whatever you like. And when you think you've read enough, read more.