It seems developing a new engine a relatively rare, difficult, expensive, and risky endeavor in aviation. Notice none of the aircraft companies make their own, right? It's CFM, PW, and RR.

But Boom has a bunch of propulsion engineer openings so it looks like they're really going for it.

The saying in aviation is "don't develop a new airframe and a new powerplant at the same time".

Sure there are counterexamples, but they have good reasons to think that this is more than double the difficulty of developing one of these parts. And that the engine will take longer.