I can't comment about the Nook but I've got a Kobo libra 2 (the later version to the h20, I guess, $10 more in Australia). 7" screen is nice for my older eyes. These take ordinary EPub books which to me is one of the main benefits. It has a USB-C port. They also allow you to borrow books through
overdrive.com in a number of countries, though they actually filter search results in overdrive so that you can only borrow books that are also sold by kobo as well (mainly this prevents you from borrowing some rarer public domain books and is morally sus) - of course you can just borrow the pub domain books off the overdrive website and then upload to the reader. They have a nice but too expensive cover that folds into a prop that works both landscape and portrait.
I have the same issue with not seeing the covers of SE books on it (kepub or plain epub no difference) - I haven't gotten around to investigating what the problem is.
Compared with a kindle (10th gen): no ads on the kobo, easier to go between pages, faster (but still goes too slowly resolving pages in large PDF books, the kindle just hangs with them), the menus and generally finding stuff is *much* better, the text appears a bit cleaner and you don't have to buy anything from Amazon.
And yes, Calibre is fantastic.