Most of these productivity gains are achievable in any Standard ML influenced type system.
The main difference between Rust and other languages with a Standard ML influenced type system is that Rust has features that can let you get executive sign off for switching languages.
what is the main advantage of Rust over OCaml for most applications in this respect?
Ecosystem. It's much easier to find libraries for Rust.
Rust is very difficult for most people, so do you think that this advantage outweighs that disadvantage? Is that really the only meaningful advantage if you’re not writing a driver or internet-facing compressor at scale?
You're greatly overrating how difficult Rust is. The wealth of high-quality software being written in it shows many people think it's worth the effort.