> You hear this "learn the fundamentals first" so many times and it really bothers me as I don't think it's great advice

Exactly. This is also why we don't teach ZFC set theory to 5 years old. We start with "1 + 1 = 2".

When they learn functions, like sqrt, we don't tell them about imaginary numbers.

This is true for ANY subject. You start at the surface, and you dig deeper. Analogies made at the beginning will help you make a mental visualisation of what happens. 99% of the time, those analogies are just false, and that's ok.

Imagine teaching to a 10 years old about quantum mechanics because it's wrong to say that the universe is made of many tiny balls...