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It’s been a few years since I’ve had to do this, but I think that (at least back then) Calibre defaulted to MOBI for the conversion. However, you could manually select KF8 (AZW3), which is essentially EPUB with a different file extension.

You sideload them as epubs and they're fine on my Oasis at least. Calibre does a good job of fixing metadata like covers.

You can find Calibre plugins to convert the books to KFX, Amazon’s native format. There’s also a plugin to recover actual page numbers rather than loc markers in the books. It’s not very intuitive, but it’s doable given the options Amazon gives us.