Another factor that you might want to consider is the user interface - what are you presented with when you boot the device, and how easy is it to get from there to doing whatever you want to do? Unfortunately, it's not terribly easy to evaluate this quickly. You need to be able to use a device for a while to get a good feel for it.
My opinion, for what little it's worth, is that the Kindle UI used to be quite good (about 12 years ago, when I got my first one), but has got worse and worse over the years. I find it now all but unusable. For example, trying to get the Library page to display a list of my collections is very difficult. And when I do so, I find that each item in the list takes an unnecessarily large space because of the enormous image on the left. A page on my 6" Paperwhite is only big enough to show four collections at a time. That UI is the primary reason that I no longer use Kindles, and have switched to Kobos.
The Kobo UI hasn't always been great, but has gradually improved over the years. IMO, it's now the best available in this country (the UK).
I have no experience of the Nook: B&N pulled out of this country in (I think) 2016.
The Onyx Boox UI is pretty decent (though not as good as Kobo's). Its biggest flaw is that its sorting on authors' names is done by forename, not surname. Its greatest strength is in the excellent formatting control that it offers within books. For the 6" screens that I favour, my Boox device is smaller and lighter than my Kobos or Kindle. I also find that the internal clock of my Boox drifts dreadfully: it loses the better part of a minute a day. (This isn't an issue if you're happy to turn wi-fi on frequently, because it does reset when connected.)
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