At what power level would a self defense device like this become dangerous?

Tazer Glove

Assuming you could craft something like this, how much power could you ramp it up to before you risked permanently injuring the person you are trying to stop?

The commercial Tazer is a dangerous device at best.
It is for use in controlling folks that are a danger to others or themselves and not to be used by amateurs.
Making modifications to increase the output can be hazardous to YOUR health.

You can’t know at all what will affect an unknown victim. Too many variables, and too many people with potentially lethal conditions that just need the slightest jolt to trigger them.

Any self-built electrical gadget could become dangerous when used on the wrong person.

Remember that it is current and not voltage that kills. If I recall the numbers correctly, only 1 or 2 milliamps is enough to get a good shock, and 10 or 20 milliamps will kill. But you had better verify that with an electrical safety manual.

It seems like what you have is a 300 volt voltage source with the camera. You would need to add to that some sort of current limiting circuit to hold the current below the lethal level, regardless of the (electrical) resistance of the victim.