Siri turn by turn directions volume too low over bluetooth

Using Maps + Spotify, the volume of Siri's voice is too low when paired to a bluetooth-aux dongle to my car.

I think I'm unable to raise the volume, because if I press volume up while Siri is giving directions, the volume of the music is the one changing, while if I invoke Siri, the Bluetooth connection switches to the handsfree protocol, Siri's voice lowers in quality, and the volume is adjusted only on that protocol.

Furthermore, iOS 15 removed the setting for Siri voice loudness in Maps.

What can I do?