On the plus side, the old MOBI format does support most of the formatting used by virtually any plin old textual-oriented fiction ebook -- although fancy stuff like embedded fonts, SVG graphics, and some complex formatting were introduced in AZW3, you probably don't need them.
Also, old MOBI is much messier inside, but that literally doesn't matter at all unless you happen to edit your ebooks (which some do, to fix "problems"
).
On the minus side, you really are limited to either having cover thumbnails or having the book in the 'Books" section, but not both.
Unless you
sideload to an E-Ink device.
...
Happy to help, by the way. Why do you think I lurk here so much anyway -- racking up ~17.5K posts.