What should I know ahead of time before going in full time on a startup idea? [I will not promote]

I'm a software engineer at a big tech company and I've been wanting to quit for a long time due to the return to office policy. I've got an app idea that I'm currently in the middle of validating and getting a prototype developed for and I'm really liking how it's turning out so far. I'm planning on tentatively quitting my job by end of year and going all in on this app for at least 6 months or so. I'm thinking that even if it fails I can still gain valuable experience and jump over to another company. Before I take the leap I'm giving myself lots of time to think this through. So far I'm planning out how my monthly expenses and health insurance will look like while I tap into my savings during those months. What else should I be considering carefully before I leave my job to go in on this idea?