One Linux you can still select the internal microphone even with something plugged in the jack port and the mic is still usable.
Whatever OS you're using, you're not magically going to regain access to a physically disconnected signal.
Another alternative is 3.5 mm to Bluetooth adapter from your headset. Though those are usually (?) male, and hence only work on a female 3.5 mm.
You can buy these for a couple of EUR/USD on Amazon from any Chinese brand. Fairphone themselves also sell a 3.5 mm to USB-C adapter. But this does not have USB-C pass-through.
These all do have a disadvantage: physical bloat / potentially making device less comfortable.