Firestick can run FOSS apps. You first turn off unknown sources blocking, and then use Downloader to sideload FOSS apps like Kodi.
Another option is you can buy a thin client with an HDMI port for like $100, velcro it to the back of your TV, and install a light linux distro on it. Or you could try doing it with a raspberry pi, or similar low powered device.
So if I get a Firestick, I can connect it to a machine running Linux and sideload apps like that? (Sorry if it’s a dumb question, I’ve never used a Firestick but I’ve been contemplating ways to mirror content from a Linux PC to my smart TV)
No, you turn off apps from unknown sources, then you download Downloader. You can then use Downloader to download apps from any internet url. You can do this all from the Firestick’s home screen.
I agree, but the thin client doesn’t allow you to view Netflix in 4K (maybe not even 1080p) and I guess Prime video too and others. Unfortunately Microsoft Edge is needed :( …or is there any news about this? I’ve settled for a Lenovo M720 (tiny PC) with Windows.
That’s the tradeoff for FOSS, though. Official Netflix apps and Edge are not FOSS, and Netflix in general does not seem willing to let FOSS browsers/apps stream in 4k.
The thing is that OP seems to want FOSS apps but stream from commercial companies, and there is a bit of tension between the two goals.