Even brake lights turned out to be not trivial. Not only need they to be switched by the brake pedal but also by the vehicle control unit during regenerative breaking. This is an active low and the obvious way forward turned out to be a no go: TIK TOK relay noise.
Silence! No more TIK TOK relay noise during regenerative braking.
Lights please! New relays, rear reflectors and repairing the rear light units.
Tested many LED replacement bulbs for combined light/brakelight. From cheap to expensive and they all are not usable (some even crap). Overall verdict: not enough contrast between lights and brakes especially at night and light too bright.
But if the rear light is too bright, why not dim it. First thought about PWM but a resistor worked fine. Thanks to Erik for thinking along.