Selfhosted

My Kobo Libra works perfectly with my self-hosted Calibre-Web, it syncs directly with it in the same way as it would sync with their online platform. You can also use both as it uses the later one as fallback.

Pretty much all the big brands work with Calibre.

I run Koreader on a Kobo Libra 2. I just connect to my OPDS catalogue on my Calibre-Web instance. It's not exactly a sync setup; it just gives me access to my library whenever I need to download something, and that covers my needs. There are several other sync options; check out Koreader's features here: https://github.com/koreader/koreader/wiki

If you like it and decide you want to it, go through the list of supported devices and see what sort of sync capabilities are available for them (support for Kobo devices seems to be the best/have the most options).

On the Kindle, you can email yourself e-books.

There's this company, which makes ebook readers that don't seem to be tied to any particular vendor:

https://shop.boox.com/collections/all

They have so many models, though, that I have no idea where to start with them.

I have a Kobo Libre and with KoboCloud[1] it syncs automatically with Nextcloud and other cloud providers. It’s awesome.

[1] https://github.com/fsantini/KoboCloud

I use an iPad. Apple doesn't seem to care where the file came from, they just make it readable. I was somewhat mad about it, as I have never been an Apple fan, but the actual experience of using the iPad as a reader converted me.

I just use Kindle for my calibre/calibre-web stack, all within the KOReader app)

I just use Kindle for my Readarr/calibre/calibre-web stack.

I’m interested as well. I hope there’s a different answer than Amazon Kindle, as I refuse to buy anything from them

I use a Kobo Libre 2 with Calibre Web to sync.

Been enjoying using my Boox Nova Air that connects to my Kavita through Moon Reader+. I just download my epub or PDF to read. That flow has been working like a charm to me.

Keep in mind that it's an e-reader, so it's black and white only. May be fine with you or you may want an iPad or an Android tablet for color and speed

EDIT

Have read also from my Kindle and Kobo library as it is an Android device and have both apps downloaded

Onyx Boox Leaf 2, and use the OPDS feature of calibre(web) to download books and read them, all within the KOReader app

Not the answer you're looking for, but I have a self-hosted Calibre server and I stuck to a second hand Kindle I got. It would be neat to be able to browse my remote library like on the Kobo, but I'd rather buy what's second-hand, cheap and readily available (lots of these perfectly working pre-loved Kindles and Kobos). Transfer lots of books at once and I rarely have to do it since I read slowly. If you use it for magazines/news/comics, then other more libre and open recommendations seem quite good.

I have a Paperwhite 2015 version that I got back in 2016 for only $30 when they had a big sale on them to unload for their new version. Looks like on eBay that 2015 version goes for $30-50 today.

I transfer books to it via a USB using Calibre. It doesn't need nor do I connect it to WiFi. Newer models might also be able to work via USB only, I don't know, but I know my 2015 works that way.

My boy pointed me to this one the other day, it’s like completely open. Does not care where you get the book from and can do books and comics ( even though this particular model has a smaller screen)

Pocketbook Touch Lux 5 | E-Book Reader | Glare-Free & Eye-Friendly E-Ink Technology | 6ʺ Touchscreen with HD Resolution | Wi-Fi | Adjustable SMARTlight | Micro-SD Slot | E-Reader in Ink Black https://a.co/d/gXY8b2e

Check out the Onyx Boox which might cost a bit more but run a version of Android.