Hi Otto,
I just wanted to report back that I had great success with this. I bought a Chromecast Audio, and after all the setup, my dad can now play music from his library on my Plex server (at my house) through the Chromecast Audio to his stereo receiver and A/B/C sets of speakers at his house by controlling it with the Plex mobile app on his Android tablet.
The Chromecast Audio seems to perform the job quite well. I noticed the Chromecast Audio was a choice for casting from within the Chrome web browser on his Windows laptop, so that could allow him to stream Youtube audio through his stereo as well. However, strangely the Youtube mobile app also has a cast button and it did not show the Chromecast Audio as a choice for casting (though it showed his smart TV). I did some searching and I think Google has just disabled that unless you are paying for a Youtube Music subscription.
In the course of setting this up and testing, I did install the Plex Windows app on a computer and noticed it was really slow and slowed down the computer - similar to what other people reported here - Plex App Windows 10- Downloads not working and laggy performance
But I did install Plexamp later, and that worked well besides lacking interface. I think I will install Plexamp on my dad’s laptop so he can access the same music that way. He would be able to also play the music through his laptop speakers then, or he could use that similarly to the Android tablet and control play through his receiver.
