I just developed a new SoundTouchPlus custom component if you’re interested. It can easily be installed via HACS as well. More information can be found in this forum post.
It might be able to help with the select_source, as it supports both the source= and sourceAccount= values that you need to select a custom source. example:
service: media_player.select_source
data:
source: PRODUCT:TV
target:
entity_id: media_player.soundtouch_10