Programming

I would just use an iPad. You can buy them really cheap secondhand.

Even old models have excellent color quality and wide viewing angles. New(er) models have ambient light sensors that pick up not only the brightness of the room but also the color temperature of your ceiling lights, which are almost certainly different to natural sunlight through the windows, and will seamlessly adjust the picture to look "right" throughout the day (if you enable that feature). You can make it even more yellow at night if you want.

They do unfortunately have a built in battery but the battery power management features are first rate and the operating system will quickly detect a permanent power source and reduce the charge level of the battery to make sure it lasts (lithium batteries don't like being full all the time).

They use almost no power, have no fans, etc.

You can find plenty of picture frame mounts to hide the cable, and there's a "guided access" feature (intended for kids mostly) to lock it down. You won't be able to close the app or turn the iPad off in guided access mode.

There are plenty of picture frame apps, or you could write your own (as a website if you're not comfortable using Xcode). Definitely rotate the photo at least every few hours, or the display will burn in. And I wouldn't put it in the sun - you want the display running at less than full brightness to further extend the life of the device. Additional customisation is available via the "shortcuts" app, which is a visual scripting/automation tool.

You could also choose not to run any app at all, and just leave the iPad on the regular home screen with a rotating wallpaper, some widgets showing the weather/etc, and maybe a couple apps to do home automation or whatever.