I'll say this: it's incredibly hard to have a full-time job and also build out a successful side project. It sounds really nice and rational when you write it out here, but it requires an insane amount of focus and basically shutting out any leftover time. Most people can't do this.

I would suggest surrounding yourself with wealthy people and keeping them updated. When you have enough progress to get them excited about it, ask if they'd come in as an angel investor. Once you get a few of these lined up, you can get enough runway to justify quitting.

The base advice is good: don't quit your job if you don't have 6-12 months of runway first. But also, your job is holding you back from the execution that will be required to build your project and get customers, so try to find a way to bridge your exit.