Signalling the car "yes, I'm in control" is a good and only solution, however, tapping the brakes is (a) definitely not the only way to signal that; (b) not a intuitive way to do that if you really want to make this signal but aren't sure how; (c) not a safe interpretation, as it's likely that people will tap the brake with other intent than the signal "it's ok, I got this".

An appropriate way to signal it would be distinct from any emergency controls and also show a highly visible signal that you've turned the system off and are in a different "mode"; even a simple separate dashboard switch with a light would be better for it than the brake pedal.