I also find e-ink devices a lot better for long reads.

Remarkable is more geared toward handwritten notetaking. If you don't need such features and just want to read stuff, the high price is not worth it.

kobo-elipsa (and the all-new kindle scribe) have a similar large size (+stylus writing) at a somewhat lesser price.

Most kindles/kobos (and others too) support koreader [1]. this "app-mod" has a superb pdf reflow [2] mode that makes reading pdfs a good experience even on smaller displays (and these come with an even lesser price tag)

So koreader is the short answer. If you already have a kindle, give it a try. It is open source and easy to set up.

[1] https://github.com/koreader/koreader [2] pdf reflow is the pdf format equivalent to "responsive Web Design".