Insert a dictation icon that would revert to the Apple keyboard microphone. It would work similar to what happens with passwords.
This is a great idea. Unfortunately only Apple can implement it. Our keyboard has no way to tell the system to switch to the system keyboard nor to the microphone within it.
Apple IS adding the microphone to the screen when a third party keyboard is active on the iPhone X. It is uncertain whether they will support iPads or other iPhones with this feature however.