I actually bought a ausustor NAS with the intention of flashing it with trueNAS or run headless. But tbh the OS does what I need rn with reasonable effort and I actually stuck the the stock OS. The backup options are not that great to use and with one press backup you can only save a single share at a time. So I am using syncthing for that.

I saw esp32 picture frames That fetch images via WiFi, but this would be a lot of DIY.

My personal favorite is using all already always on devices such as a wall mounted tablet and let them have a screensaver that rotates through is set of images that change regularly.