I agree with your general point, that we don't need insane levels of math, but I would say a college level of calculus, linalg and probability is baseline.
A basic benchmark off the top of my head:
Being able to pick up, without stumbling on the fundamentals
- what LoRA is doing
- how a RBF-kernel SVM works
- why KL and reverse-KL are different
- why using mean squared error is equivalent to MLE on a gaussian
Not saying the four above pieces are all necessary, but that you should be able to learn them on demand without needing to revisit what a basis vector is.
"Working out derivatives of arbitrary functions" is school level.