Low mic audio during calls, but voice memos, speakerphone, Siri OK

Hi everyone. I’m pretty stumped with this iP7+ I have. It was brought to me with mic issues during calls and I’ve taken the following actions:

• Replaced the lightning port assembly several times.
• Ensured there are no obstuctions in the mic channels. I’ve even removed the lightning port assembly from the body and tested the mics exposed.

The symptoms:

• During calls, the sound volume is very low and sometimes cuts out altogther. The sound is normal volume when holding base of phone 1cm from mouth. When I hold it this close, the sound just seems to jump back to normal.
• The right bottom mic doesn’t make any sound when I scratch it with a spudger, although the left one does (noise cancellation?)
• Blocking the rear camera mic brings the sound back to normal volume.
• Turning off noise cancellation in Phone settings does nothing.
• Camera videos sound normal.
• Voice memos are audible but have loud background/interference noise.
• “Hey Siri” and Siri has no issues responding.
• Calls on speakerphone sound normal.
• iOS 12.3.1

I have noticed that no sound will come from the loudspeaker unless the connector is pressed down, which makes me think it’s an issue with the connector on the logic board itself. Other than that, all functions on the lightning port cable are fine. Any thoughts very much appreciated!