crappy flare tool

The type of tubing has nothing at all to do with flare angle. It is the FITTINGS you are using the tubing with. Inverted flare/ common SAE flare is 45, JIC/ AN is 37 This applies to any type tubing

Maybe you can anneal the tube first
actually, yes it does. Stainless steel is actually a tad more brittle, which is why it splits. which is why it requires a single, 37 degree flare. I worked making lines and flaring them for years. I'm now working with them again for the military on the engineering side.