The goal of the continuous playback is to continue playing music after the song/album you chose has finished. The shuffle/mixes options in different sections of Symfonium doesn’t enable you to choose the first song/album you want to hear.
If you want an example of how something similar is done in another app, look at Poweramp on Android. They have implemented advanced repeat and shuffle options which allows random continuous playback of songs, albums and even more categories. For that to happen, you start playing the song/album/etc you want to hear and then enable the “Advance category” option for repeat and the “Suffle just categories” option for shuffle. When the song/album/etc you chose in the first place finishes playing, Poweramp will play another random song/album/etc.