Its really easy to make a custom rom but hard to do serious "real life" stuff; companies don't want to make it easy. To most regular users, if they cant download apps from the google play store, and they can't use venmo\cashapp, then the OS is dead in the water from day 1
You need to solve the 3 player problem before you even ask for money: getting device manufacturers in even though you have no operating system, no devs and no users, getting devs even though you have no operating system, no devs, no users and no devices, getting users even though you have no device, no operating, no devs and no apps.
You need an MVP that shows promise towards all the above if you seek money.
This is like taxi on demand app business or the takeaway delivery business but with more players and with a higher minimum funds requirement. Plus the fact that unlike taxi apps or takeaway apps, choosing an operating system is a zero sum game so you are competing in the most direct way against well known and well established brands like iOs and Android who are funded by the richest companies on earth. Unlike Uber vs Lyft, where a user can install both and use both, your battlefield only has one victor. And given that other companies with more funding that you will ever see in your lifetime still failed, you have a virtually impossible task of explaining (before they even consider giving you a single cent) how you are going to be able to capture market share with your own solution to the 3 player problem.
Nokia and Microsoft only understood this right at the end: to avoid losing in the mobile os market, you need an ecosystem. Miss any of the elements and it all crumbles. Read Elop's memorable Burning Ship note on the final days of Nokia.