Sounds like some app is messing things up once it’s got going.
Could you try using Safe Mode? This sort of temporarily simulates a factory reset but without the pain.
If Safe Mode allows stable microphone functioning then I would advocate making an exhaustive list of apps that are allowed access to the mics, and remove their permissions - one by one so that you can find the culprit. I suppose you could also work the other way: withdraw permission for all apps and then gradually grant again.
To list all apps with mic permission,
- go to Settings > Apps > See all apps
- choose an app that you know has mic permission
- tap “Permissions”
- tap “Microphone” > See all apps with this permission.