First, I would recommend you continue to use your Nexus for PDFs and the odd ebook that's not compatible with whichever e-ink reader you buy.
Next, I was going to recommend a Kobo Aura because it meets most of your needs, until I read the line about reading in landscape mode. My husband reads in landscape so I had to purchase a Kindle Paperwhite for him. Kindles lack SD cards so while you can probably fit all your ebooks on one, you will run out of space with large PDFs.
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