1. You're hoping for UBI so that you can quit the work force and get a PhD, so you're going to (at least temporarily) cease to input money into the system, and start to take money from the system. That seems like effectively twice the cost. Where does that money come from, if you and other people have that same idea? This is the part of UBI I'm struggling to comprehend. There's the idea that you will come out of school "more valuable," but that's speculative vs your current position which is actively adding value (albeit less than what you feel you're worth).
2. Why do you need UBI in order to do this? Are you making no money beyond the absolute minimum you need to live with your current job? Couldn't you potentially save some away and then do the "nights and weekends" sort of school that some people do? To your very last point, you mention having to hope for one of your startups goes big, but could you take a more "steady" / traditional job and make a decent amount of money that way?
Unpopular opinion: this strikes me as exactly a case against UBI. You're taking an active, and perhaps more importantly, tax-contributing member of society, and incentivizing them to stop being one. "Innovation would move quicker" is a great idea, but I don't think it's all that much of a guarantee. It seems likely to me that it would continue near the same pace. But, that's also just speculation / without citation.