Aircraft are hard, you need an army of experienced engineers, like thousands, and hundreds of millions of dollars of capital and resources to design a 21st century aircraft. It doesn't matter if you're Elon Musk or whoever, that's the state of the art nowadays. You need to run numerous (supersonic!) wind tunnel tests of a design (each test spanning months, with year long lead time before testing, and months of analysis and design in between tests). There are only a few facilities in the world (like the 10x10 at Glenn Research Center) that can provide such wind tunnel testing. Structural and aerodynamic work will take years of iteration to meet basic design challenges. Actuators, flight computers, air data sensors, etc... need to be chosen with very careful forethought via trades with big name suppliers, and lead times for these things can be 9 months to 1.5 years easy. The engine is as important as the entire airframe its self, and there are only three engine manufacturers in the entire world that could make an engine for a plane like this. One would need to be designed from scratch from one of these manufacturers and would easily take 3-5 years! Oh, and they would need a ton of capital and thousands of engineers to do it too. This doesn't happen from a scrappy looking start up with random engine parts sitting in a room with a whiteboard. This is a joke, investing in this company is a sure way to throw away your investment. Sorry, that's the cold hard facts.
Yea, spacex managed to succeed, at least so far, but look how big they are now and how long it took them to get where they are today. And I would argue that spacex's engineering challenges are easier than those needed for a 21st century supersonic jet transport.
Sorry, that's just the state of aerospace engineering today. Major new aircrarft are only developed by the big boys with the experience and resources to make it happen over a 10-20 year time frame with continued but small investments in R&D, and incremental leaps in technology.