You may try to quit your job gradually -- talk to your boss and explain that you cannot dedicate as much time to your job anymore and can work only ~3 day per w

You may try to quit your job gradually -- talk to your boss and explain that you cannot dedicate as much time to your job anymore and can work only ~3 day per week (with reduction of pay of course). When (if) your new business really picks up -- you would be able to make safe transition to your full-time start-up involvement.