Do you know if there is any open hardware alternative for products like Roku, Chromecast, Apple TV etc. ?
Not quite open hardware but:
There are a few others. It seems like devices with Amlogic S905D3/S905X3/S905Y3 are generally supported. Still I'd pick one that has mainline builds by an Android OS project.
Depending on what you need, you might get away with any old small PC (like 1 litre office PCs or old thin clients), or an SBC like Raspberry Pi.
For Operating system, instead of Android, try LibreELEC, or any Linux distro starting straight in to Kodi could work, for Firefox in Kiosk mode opening your Jellyfin.
Bazzite is a version of Fedora initially started as a steamOS alternative for the steam deck. So if you install the Deck version of Bazzite it boots straight in to Steam Big Picture intended for the TV.
KDE Plasma also has a Plasma Big Screen version, but I'm not sure how ready that is yet, or if it's easy to install.
I was wondering about this as well.
