If you're comfortable with microcontrollers (esp32/arduino), I can definitely recommend Inkplate. I found them when I was making a similar setup for my parents, and they have various sizes up to 10" and up to 6 colors they can display.
You can either just get the module, or buy with a battery and mountable case already attached. I think all of the models are also available via Digikey and Mouser if people don't trust random websites.