> There is a lot to be said for understanding types as a kind of test, but performed at development time rather than runtime. And valid for all possible inputs, not just some specific ones.

Maybe you know this, but this is a well known correspondence (as in academically from decades ago, but barely mentioned in industry): https://en.wikipedia.org/wiki/Curry%E2%80%93Howard_correspon...

The types you write in code describe program properties to be proven and the type checker tries to create a proof that those properties are true for all inputs and outputs. Tests as you say only check a small number of inputs/outputs in comparison. It's very similar to mathematics where checking if a conjecture holds for a few values is a poor substitute to a rigorous proof.