I use a Kobo Libre 2 with Calibre Web to sync.
Do you use koreader? I primarily want to be able to switch between reading on my phone and my kobo.
No, I just use the built-in reader. I tried a couple of additional third party ones and my battery drain was pretty bad. Might have been a me problem though.
I have a Kobo Libre and with KoboCloud[1] it syncs automatically with Nextcloud and other cloud providers. It’s awesome.
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).
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.