Kindle is only for Kindle... it's a closed doc, reader and sales system. The doc format has evolved over time and was originally based on MOBI, but now is a more proprietary format.
EPUB is a universal, open-standard format. Unfortunately, the standard is so open and an enforcement entity so absent that it's become a huge headache. Every tool and every e-book seller has their own variant file and reader quirks and it's impossible to have a truly universal file, at least if you want to maintain any semblance of formatting or structure. (They will all display "plain text" about the same.)
Thorium is a reader re/developed by an entity that does care about the EPUB standard and doesn't have the desire to implement new, cool features on their own. So Thorium (evolved from Readium) is about as close to a standard, standards-compliant reader as exists. If your doc works and looks right there... frankly, any problems on other readers are those other readers' fault.
While fixed-page EPUB is "easy," it's also a lousy use of the tech and IMVHO largely obsolete and a misuse of the medium. (You want fixed pages? Use PDF.)
Reflowable EPUB is not an easy medium to master, and the amount of outdated, misguided and just plain wrong information and advice out there is titanic. After a number of years and many attempts to engage the various communities at a professional level, I've given up and written it all off. (I can tell stories...)
If you want to do professional EPUB and Kindle, using a modern, standards-based approach and starting with ID... see below. 🙂
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