[ps67]

I can't say anything about Libra or Oasis because I don't own them. But if You prefer Oasis hardware You can sideload books in the better kfx format (ligatures, hyphenation, kerning) with Jowell's calibre plugin of file conversion. A vast majority of books can be converted in kfx, You can try with some books an see.

It relies on Kindle Previewer, so it doesn't work on Linux. If You can't or don't want use it You will use azw3 format that can handle soft hypens with another calibre plugin (hyphenate this or polish book)* but has no ligatures nor kerning.

About metadata: kobo is considerably better in its interaction with calibre, because with a Kindle You have to create collection by hand, and it doesn't handles series information (the only thing You can do is to create a collection including the books of a series, but then You won't have the possibility of ordering them by series).

Some poster are worried about future of sideloading in Amazon's device: but who can say?

*If I remember correctly, for some mysterious reason azw3 can handle hyphenation (no need of soft hypens) in Russian and only in that language.