It also sounds like an unhealthy situation: imbalanced team, sketchy uncertain compensation (cash/equity), low respect, nothing to help you with where you need to grow (which includes both technical peers and having the confidence to interview elsewhere).
No point in trying to rush something to 'complete' and only then leave. Software is never done. They'll need to get more engineers in any case, those new engineers should do it the way they'll need to.
Respect anything you've signed so far; don't sign anything new; don't directly compete or take anything you wrote for the current unpaid project with you.
Though you say the CEO is nice, it also sounds like you're being a little bullied, and expect to get more 'temper' from people when you leave. Brush it off; it's their situation and emotional reactions, not an objective analysis of things. Don't get into a long discussion, don't accept a 'counter-offer' or promise of change. Go elsewhere definitively.