Some more testing. It seems on phones, the default audio route is to the earpiece of the phone and not the speaker. Somehow changing to the combine headphone option and back gets rid of the earpiece route. Another workaround is to select the speaker. This issue is only present on iPhones. Not on iPads as they have no earpiece. Only speakers. The issue does also not occur when connected to headphones or a usb audio interface.
I think there is a way to fix this and default to the speaker route when no other audio gear is connected.
Regarding the ducked output of other apps. I can't reproduce that issue. Playing a song in ST3 then switching to Youtube or Spotify and playing something in there will play it at the same volume. No drop in volume. @Chemabazan how can I reproduce the issue? Do you have step by step instructions?