I don't think there is anything that meets your requirements. I think the closest are the Kobo devices. These allow collection management from calibre. The books can be put into folders, but these are not used by the interface. Someone has written a tool to generate collections from the folder structure, but I don't know how well it works.
The Kobo's to look at are probably the Aura H2O and Glo HD. The H2O has a 6.8" and 1430 x 1080 screen. It isn't Carta, but is still excellent and is waterproof if you read near the pool. The Glo HD has a 6", 1448 x 1072 Carta screen. Both have 4GB internal storage, with the H2O also having an external micro SD slot. Both have an internal micro SD card for the storage, so it is possible to open them and replace the card with a larger one.
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