For #4, I don’t think scaling should be a major issue as the tooling industry has been manufacturing at scale for decades now. The only real difference is the customer
I think the requirements at the semi level are quite different than what the tooling industry will be able to produce. The silicon carbide layer thickness probably needs to be controlled at least to the 10 nm level and the film quality will need to be quite high for optical applications. There is some use of silicon carbide in semi in high power electronics, but I don't know how well that transfers to optical quality either.