I have managed to get my iPhone to properly randomly shuffle all my saved songs in my library (there are 1000’s of them). I sort them by song title, press ‘shuffle play’, and have the randomiser turned on. HOWEVER, this is only working properly when playing on the iPhone. When I cast to any of my Google Chromecast Audio devices, the Spotify app starts randomly playing a smaller selection of songs that start with the letter of the last song played before I started casting. So perhaps a Google/third party constraint on the Spotify app? Very annoying, and needs fixing. I hav also posted on Google Chromecast forum.