Extremely poor mileage of 11.6 km/l and other assembly issues

I have purchased the car on 28th Jan-21 and ran 4,000 Km and experienced extremely poor mileage of 11.5 km/l and some major assembly issues like break failure, engine acceleration, internal noise, OVRM issues, which the service team claims to be resolved . The poor mileage of 11.5 is sucking my blood and seriously thinking of selling of the car.