>>105839315facebook marketplace has plenty of e-ink displays.
you cannot load arbitrary ebooks onto the kindle, all the books need to be on the amazon store, and also there has been some drama with amazon removing ebooks remotely and removing the ability to store ebooks on a computer (offline I assume?). And I think amazon has locked down on jailbreaking, it's pretty difficult now.
The HD clara color is the new model that has color (It's probably worth it because it's not much more expensive compared to a B/W display with the same pixel density/features, the $85 kindle has no waterproofing, and no "warm" backlight).
Older models are like $10-20 on facebook marketplace but note the models might be like 10 years old, but it's fine, the pixel density and contrast might be slightly worse, probably not water proof, and no backlight, but that's not really going to give you eye burn or anything.
you can pirate books from libgen.is and it should just work I expect?. However, the kindle might be better in terms of cost because most people who buy an e-ink display end up giving up after 1 book, so the premium of kobo being able to pirate books isn't worth it.
I think ipads have apps (or websites) that are made for pirating mangas, and I think none of the e-ink displays would support (I think there are e-ink android OS's but it's probably expensive), so I think the ipad is just way better for mangas, maybe you might prefer the kobo clara color (if you dropped the $ for it) but chances are the ipad is still going to be a better experience because the colors / contrast / app.