Starting your own stuff requires a just try attitude.

You should try to fit it into your schedule, instead of hoping your schedule can fit it. Worry about not having enough time after you start it running. Can decide if you want to do it full time or allocate more to it at a later stage.

I don't see the dilemma to be honest. If you feel good about the new job, just take it and start your project too. If you don't feel good about the new job, then wait for a better offer or role.