How good is a ReMarkable as an ebook reader? I don't care that much about the writing, but having a large size ebook reader that can display A4 pdfs in acceptable quality would be a game changer for me...

It sounds pretty basic from what other commenters are saying, but you can install koreader, which is outstanding (and also supports PDF, DjVu, XPS, CBZ, FB2, PDB, TXT, HTML, RTF, CHM, EPUB, DOC, MOBI):

https://github.com/koreader/koreader