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The Kobo Libra 2 has most of what you want, if you can compromise on the 7" screen.
It supports EPUB/PDF/HTML/MOBI, drag and drop sideloading, 50+ hours of reading per charge (depending on frontlight), and it supports large libraries. You don't actually need to connect to a Kobo account to use it, if you edit the configuration file the first time you turn it on.
I've never used an Onyx device, so I don't know how good the Boox Darwin is, but I guess you can compare its features to the Libra to see if they outweigh your desire for a 6" screen. I only mention the Libra since you said you could be flexible on the screen size.
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