I think eink is likely as good as it's getting. The screen is a high cost so I don't know if the 10" to 14" will get cheaper.
I'd like to be able to afford multiple sizes, or have a device that pulls out from A5 to A4 or longer. Technically possible.
Only some radically different tech can add colour or video (Mirasol seems to be over hyped, too high royalty and too expensive). LCD and OLED take too much power and lose the biggest advantage of eink, working with ambient light. I hate the front lights and only use them if there is a power cut or I'm stuck waiting for someone in the car after dark.
A 10" tablet seems to be minimum for a wide range of PDFs. Except battery life is poor on mine because it's x86-64 Intel Atom with 32 bit Win 10. I did try Debian on it which allowed 64 bit Linux, except you need a custom script or touch screen is wrong when you rotate

I might try again if I need a lot of portable PDF reading.