LibreELEC is a Linux option if you want a full-featured device. LibreELEC (or Kodi, which is the application that actually does the heavy lifting) has both Jellyfin and Disney+ add-ons (although I can't personally vouch for how well they work).
Android TV is also available for the Pi and is another fine option.
You may be interested to know that while they're Nokia branded, the brand is just being used under licence. The actual manufacturer is StreamView, from Austria.
Yes by a company called HMD. They actuality continued the vision of Nokia and I think what they're doing is pretty good. They make a lot of feature phones and I owned the Nokia 8 which was pretty good for the money.
Not a proper alternative really. Some of your own set up too, you could buy a laptop or small form factor PC? Lots of great Linux distros to get on there.
I have done that in the past, using a raspberry pi. I was using Plex (Rasplex) back then. The main problem was convenience, mostly with the possibility to have proper remote control from the couch.
For just myself, this could possibly work, but it’s a non-starter for the non technical people in my house. They need a remote control on the couch and the ability to easily open different streaming apps with that.