Dynaframe or the Pro version on Patreon. I've been using the Pro version for about 8 months and really like it. I have it setup with an RPi4 and a 15.6" portable monitor. I pointed it to my NAS pictures folder. It doesn't rely on any services beyond a local network. I'd recommend using Ethernet instead of wifi for stability. Only issue I've run into is the web UI to configure folders and settings can fail to load for some reason. I think this is a bug though, and I should grab the latest version to see if it's fixed.

I've been using a NixPlay frame for the last five-ish years. They're doing fine, and their frames have an SD card slot.

I recommend it.

I know you said no proprietary, but frameo is a digital frame app/service that is leveraged by various manufacturers. I recently gifted my parents some no name picture frames that use frameo, and they're extremely happy with them. I can add pictures from anywhere in the world, and multiple people can be added as contributors. It cost me around $50

Forc Digital Picture Frame WiFi 10.1 Inch Digital Photo Frame, Electronic Picture Frame, 16GB Storage, Auto-Rotate, IPS Touch Screen, Easy Setup and Share Photos and Videos via Free App from Anywhere https://a.co/d/9LJYELQ

This is the one I bought. Searching for frameo on Amazon will show a wide variety.

You can probably slap something together with a Raspberry Pi and connect it to a low-ish power TV

Chromecast has a digital picture frame built in that uses Google Photos, if you trust that to stick around. You can plug it into any HDMI display.

I'm guessing something like Roku, Amazon, or Apple has a similar feature.

Get an old tablet and put a frame around it?