One of the difficulties of breaking up a large crate is the Orphan Rule. I got bitten by it once. I had some traits and when I broke up the crate suddenly some types but also the traits were in different crates each.

I wanted to separate the type, the trait and the implementation of the trait in different crates. I was forced to use newtypes but newtypes need a lot of boiler plate. It was painful.

In my opinion this is a larger barrier than having separate Cargo.toml files.