I splurged on the 13.3" Onyx BOOX Max Lumi for largely similar reasoning: I do a lot of reads of older materials with poor-quality scans, and wanted to be able to read as many of those at full-screen rather than zoomed resolution. I also had concerns with the size of the device.
I'm happy with my purchase, though with experience, I'm happy to suggest trade-offs.
Onyx's product line is based around 7.8", 10.3", and 13.3" e-ink screens. Those are what their vendor offers, so it's what Onyx bases its builds around.
My previous experience was with a 9" emissive (OLED) Android tablet, which saw principle use as an e-book reader (the only application to which it's well-suited), with portrait-mode presentation of books and articles working fairly well. It was hard to read in bright light, with overhead lighting (screen glare), or, of course, outdoors.
The jump to 13.3" e-ink at 220 ppi makes even low-quality scans of small-print materials readable at full size (without zooming). There are a few specific cases that really impressed this on me. Many books can be read landscape mode, 2-up, which is also good.
The device is large, about the size of an A4 or Letter-sized paper tablet. This is just about at the maximum size for comfortable in-hand viewing, and you might prefer a smaller device. It packs easily into my courier pouch.
If you don't wanto to spend the $800 (over $900 when you include stylus and cover), you can shave a a few hundred on a smaller device with some compromises in reading convenience. There's an in-page navigation that will scroll either in "reverse-N" or "Z" mode, for articles or comics, respectively, dividing the page into four regions (a 2x zoom). This makes virtually any material readable, at a cost of not being able to see the full page in a single glance. Even with a 10" or 8" device, you'd likely only have to engage this for a portion of your reading. Most books are readable at 100%, and even many articles. You can also trim margins to increase the size of the actual text on the page.
Battery life is quit good (I get a couple of days per charge, but use the device for much of the day, what I'm spared is any battery anxiety), performance is quite good, the display is delicious.
The notetaking feature wasn't something that attracted me to the device but it has turned out to be surprisingly useful and appealing.