Definitely get the ereader you like the most, and then use the Calibre app--which you can get a lot of help with here on the forum--to convert your ebooks to whatever format you need.
It's good that you've tried a few readers hands on; knowing you prefer a larger screen size is really helpful. In the dedicated e-ink reader world, that narrows it down pretty quickly to a handful of good choices: the H20 you are considering, an Oasis, or a Kobo Forma. There are a few others but not that I could recommend.
IMO there are two nice things about Kobo devices: they are flexible in handling more ebook formats (which can save you an extra conversion step when buying ebooks), and they are built on the Linux OS, so they are easier to customize and install 3rd party apps. In comparison, the Kindle devices to me, seem to have a bit more refined software, hardware, features. You really will be ok with any of these devices though.
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