Out of curiosity, I tried to set Bluetooth audio speaker as default device in Plasma settings:
The result has surpassed all expectations.
It works perfectly in Solus Plasma. Sound from all audio sources, without any exclusion - media players, notifications, browsers - is automatically redirected to external Bluetooth speaker. There's no need to manually indicate it in each single application.
Yes, the default device is dropped back to internal speakers after reboots - but only because Bluetooth connection gets interrupted. Once Bluetooth connection is re-established, Solus Plasma automagically (!) sets Bluetooth speaker as default device again. Without a need of any manual intervention.
Thank you guys for helping to find out such a nice feature in Solus Plasma 🙂 .