My suggestion would be the DR800SG with the 3rd party FBReader reading app. The 1st screenshot is from an iRex Iliad using FBReader at 36-point. This is an 8" screen, just like the DR800SG. The image will likely seem larger on a PC monitor than the device. Someone running the DR800 emulator may be able to produce a similar screenshot closer to how the DR800 will show this.
It sounds like you may be able to use a smaller font than this, FBReader allows you to choose from lots of font sizes. On EInk screens, making a font bolder can make a big difference in its readability. Most fonts appear heavier at large sizes, but FBReader can auto-bold everything if you want. FBReader is available for a Windows PC (or a Linux Desktop), so you can try it out before buying. The DR800SG version is
exactly the Linux Desktop version (very similar to the Windows version). This means you will need to use the stylus on relatively small icons to do anything except turn pages, but when reading an ebook turning pages is all that is needed routinely. Choosing the ebook from your library still uses the DR800SG's navigation, rather than FBReader's library software.
FBReader is an add-on for the DR800SG. So it isn't as well integrated into the device as the built-in readers. Pocketbook and Onyx include FBReader in their standard software. They only have 6" readers at present, although both are planning larger screened devices.
If you want to checkout the Sony PRS-900, then the
Sony eBook Library for Windows or Mac can give you a good sense of what you will get. The 2nd screenshot is from the eBL with the XXL font. The text pane is 600x800, which is standard for 6" EInk screens, and the PRS-900 is 600x1024 (exactly the same width but taller). Once again this will be smaller on the actual device than your monitor.