Okay, thanks for the feedback.
At the moment, I am switching between the internal phone microphone input and bluetooth speaker output. That seems to work reasonably well, but not ideal.
Basically, I stay on category AVAudioSessionPlayAndRecord and switch the input port back and forth between HFP and the internal microphone.
This is not ideal, but it seems to be the best way to get it to work.
Otherwise, OE struggles recognizing commands using the BT microphone.