A2DP doesn't work simultaneously with HFP (i.e. "with the (bluetooth) mic on"). That's not Linux limitation, that's Bluetooth limitation and other OSes - Androi

A2DP doesn't work simultaneously with HFP (i.e. "with the (bluetooth) mic on"). That's not Linux limitation, that's Bluetooth limitation and other OSes - Android, MacOS, iOS, Windows - behave exactly the same. One solution is getting a wired mic.