I worked on a number of digital photo frames, including on ones like that.

E-ink is not the most suitable thing for it in its current form.

First, large size EPD cost an arm, and a leg, and custom one costs even more.

Second, there are no 8 bit EPD controllers as of now a mortal can buy, which means grays look very bad.

Third, there are inherent physical limitations which make increasing gray resolution of EPDs quite hard. The precision you want, the slower would the refresh rate be.

If somebody is old enough to remember, (saying this as a 30 years old is indeed amusing to myself) the last active matrix monochrome LCDs had 8 bit, and more of gray resolution, but they are all discontinued now, or cost an arm, and a leg as "medical imaging" specialty.