What do you mean with OS alternatives?

Instructions for avoiding sign in, and side-loading etc

I've had good luck with this. I buy books, and occasionally go to my library and download the books to my desktop for the day I change devices. Adept helps with the DRM stuff, and knock with the even worse ASCM DRM crap. Manage it all in Calibre.

Some books I move to a reMarkable and read them there; technical books, mainly, because they're easier to read in large format - code examples, formulas, etc., are better on a bigger screen.

Can I transfer books though SFTP or something, or just USB?

Yep, it's probably easier to get an Android device and install readers on it than to try for a prepackaged FOSS reader.

I use several apps on my Android phone, but mostly Kindle (for Kindle, duh), PDF Reader (for PDFs, duh again), and Lithium (mostly for EPUB but pretty much everything else, too). I get most of my e-books as DRM-free EPUBs and PDFs.

Always amuses me a bit when people say Kindles don't support EPUB, since I've been stripping DRM from my books and storing them in Calibre (enabling transparent conversion between EPUB and Amazon's formats) for thirteen years without a hitch. You should be doing this on any platform if you want to keep your books.

It's beyond me why anyone who so much as knows what FOSS stands for wouldn't do the same.

only offers partial support for EPUB. You have to go through a whole spiel to get an epub file to your Kindle, whereas with most other e-readers, it's just a matter of copying a file over to your device

Have you read the Amazon page you linked? Kindle supports epub now, nothing on that page indicates that it is limited in any way except file size. Also, getting an epub on your kindle is no different than any other supported file. They dropped support for mobi and azw, both Amazon specific formats.

thanks

That table doesn't contain any info on FOSS compatibility.

Kindle devices are nice but not at all FOSS, and not very open either. Although you can sideload books, EPUB files are still not directly supported, you have to convert them. Converting is easy with Calibre but it's still a hassle that is not needed on any other ereader.

There's a vibrant jailbreak community on MobileRead, however Amazon keeps blocking jailbreaks.

After my Kindle died I got a Kobo instead. Costs about the same as Kindle (maybe slightly more?). Still not fully open, but supports EPUB and its MobileRead community is just as vibrant (and Kobo doesn't block you from doing this).

Yeah Booxs' are nice because they run Android. Gives a lot of freedom.

NOTICE: The PineNote is an experimental device. PineNote software is still in it’s infancy and therefore it is ONLY suitable for experienced developers. At present time, there is no default OS for the PineNote.

Yeah that doesn't seem like a good option for most people, especially considering they're charging $400 for a product that can be considered a prototype at most.

Kobo has ways around it. Especially if you manage to get certain models. - certain variants have the entire Linux based OS installed on its SD card - you can remove it to edit the internal sqlite database with an SQL editor to bypass the login requirements. Other models are supposed to have methods to just not use a login. (Though I've never been successful at that)

Also, to save battery it usually doesn't connect to the internet unless you tell it too (sync update or whatever)

I have the Kobo Sage, while it doesn't have a removable SD card, and I did have to create an account initially. It doesn't require you to use it - you can plug it into a PC and just copy any ebooks, cbz comic files or PDFs you want to read. But I chose it for one reason: it's one of the most stable models for running KoReader, a custom interface that adds some interesting features. Including support for direct OTA syncing with the Calibre eBook management and editing software. All Kobo devices has a hidden folder that can be used with a koboroot archive file to flash stuff into the internal file system.

Running this software literally kills the original interface, (meaning it can't spy on you) and gives you full root access via an inbuilt terminal emulator. The fact it gives you access to full internal storage, means you can retroactively bypass the account requirement system, as their operating systems are almost completely identical across models.

Before that I used to own an old Clara HD (the screen broke, highly advise buying a cover for any e-reader) and did the SD card trick to bypass login.

There's a huge community for hacking most models of eReaders for better and more uniqur experiences. Theyre low security ARM devices, way easier to mess around with than an android phone these days. All Kobo devices have single account Linux distros - meaning everything is running as root.