The reason you want to use the Schnorchel is you dont have to run on the surface, which gives off an even bigger signature. If you get a radar warning message while running submerged on diesels just lower your schnorchel and go to batteries. You can cruise a long way on battery power, keep your speed to around 4-5knots. Run this way for 30-1 hour, then simply extend your schnorchel and recharge what battery power you used, repeat as necessary, and you'll never hace to surface!
to answer your question about maximum crush depth are you playing with SH3Commander 2.7 or higher?
If you are then if you have the option for randomizing your crush depth this could be why you starting taking damage at 300 meters. i have found from personal experience while plaing the typeXXI that 300 meters is the limit any deeper and you don't know what to expect! Usually at around 250 or so meters you have a good chance of finding some thermal layers to hide under if trying to avoid ASW ships.
