>Essentially, PDFs force a certain page size on you. ...

Not necessarily. Sony's early readers had automatic reflowing of text if you chose a larger font size. It worked really well for fiction or any other type of text without pictures/diagrams.

For modern e-readers, converters like the free Calibre software (https://calibre-ebook.com), will allow resizing & reflow of text in PDFs to a size more usable for any device.

>Sony used to make 9" readers ...

There are refurbished Sony PRS-900 units available (e.g. eBay) for around $50 and, unlike most e-readers, they have user-replaceable batteries.