The reason I wanted to see if I could get that Sony hardware running on some other screen (besides the re-use angle) is because Android TV gets you access to the first party Netflix and other streaming services with the proper UI. Netflix-via-Kodi is not really a very good user experience presently. You mention open source Android TV - is that something I could run on other kinds of devices? Can you get the Netflix etc. apps on it?
If you go fully open source on any device (Android TV or not) you lose full levels of Widevine (or whatever DRM) support. Apps like Netflix work on open source builds of Android TV but you won't get high quality streams as a result. I think Disney+ was one that refused to work at all, the rest seemed to run. So if you're doing "open source" for purity reasons those are the types tradeoffs you have to make regardless of the hardware or software you choose.
OTOH if you're just looking for a decent unrestricted first party external Android TV box that gets updates (old model is 6 years running on updates now) and has full play store/app support the Shield TV is $150 new and will run all of the apps at 4k 60 HDR out of the box. It can even be officially unlocked and rooted but again some apps will stop working (e.g. Disney+) if you do that. Shield TV Pro is $200 if you want additional RAM and built in app storage. Comes with a nice remote too.
Under Raspbian? It seemed to need a lot of fiddling to be usable so I went with the OpenElec image. Later switched to OSMC, still working great on a 1gb RPi 2.
Shouldn't it output digital audio over HDMI?
At the "more money but I just want a device I can run my own software on that works out of the box" the Shield TVs are great, you can throw an open source Android TV on it (not that the branded one is all that bad tbh).