I can only speak to the reMarkable. I like it, but it's definitely a note taking device that also has some basic support for reading documents. As an e-reader it's not anywhere close to a Kindle.
Kobo is also an e-reader company like Amazon and so their ePub support is fantastic. Boox's is OK but since it's Android you can literally use any ebook app you want including Kobo and Kindle so it's not a huge deal. The PDF reader on Boox devices is very good too with features like autocroping of margins.
Remarkable 2 doesn't have a backlight. Kobo's software is fine (mostly), but the design or manufacturing seems to be a notch below. On my Kobo the touchscreen is enragingly inaccurate.
And, direct access to the Amazon library is not an insignificant advantage.
What do you mean by the kobo screen being inaccurate? Is this for the stylus of the touch itself? I was considering buying a kobo but this comment scares me a bit now.
I've been using an Elipsa for about a year with a Renaisser stylus - have not noticed any inaccuracy but the writing can sometimes be laggy. I really like it, all in all though - all of my work notes tend to be on it.
I love the Onyx as it runs full Android. Also its built in apps and ink settings have a slightly steep learning curve and what seem like weird UI decisions at first, but turn out to give a lot of control over the experience.
I used my reMarkable 2 tablet all throughout university (I only recently graduated). Searchability is an area that e-ink tablets can surely solve, although the reMarkable's software wasn't particularly helpful in this regard (perhaps it has improved now). You can only convert 1 page at a time using their OCR and they only recently added the ability to tag documents.
My favorite feature was actually simple: I don't have to erase everything to rearrange the layout of my notes. Being able to simply select a portion of the page, resize, and move it was killer. Another great feature is simply not having to lug separate notebooks around. The ability to organize and sort documents was very helpful.
I've heard Microsoft Surface pens are compatible with the Kobo Sage and Elipsa.
Why did you pick a "Renaisser" over the standard Kobo stylus?
I've got a Sage myself and have toyed with getting a stylus, so if you can offer a compelling recommendation over the official stylus, that would save me fifteen bucks.
The Renaisser that I picked, the 520C, was Microsoft Surface compatible. It has two advantages: built-in usb-c charge port (vs. the weird "aaaa" batteries the stock and others take), and a felt-like stylus that felt way less slippery compared to the stock stylus. Edit0: also, unlike the stock Kobo stylus, the nib seems much less prone to wear - the former wore down in about a month while the latter is still going fine months later.