Even if you find a safe resin.

Might there be microscopic layer lines? Or other unknowns you're not familiar with? Making 3d prints that can be cleaned is non-trivial, maybe there is a surface finish involved, etc.

Also how do you know your design is correct? Won't cause your teeth to move? A 3d scan doesn't mean you know what a mouth guard should look like.

All of a sudden, having a product that's made with a vetted process is pretty attractive -- and 600 USD seems like a bargain.

What's the cost of having your teeth fixed, if they accidentally move? (Not to mention the discomfort, which can be considerable)