Yeah, I imagined Bats might be too limiting for a complex project such as yours. But in principle, it is simply a command runner that minimizes some of the boilerplate commonly used for testing. It's not even Bash-specific, and can be used to test any CLI program. I quite enjoyed using it for small personal scripts that are not throwaway, but don't need to be rewritten in a "proper" programming language either.

That said, migrating a large and purpose-built test suite to it would have no practical benefit, but it's worth considering for your next greenfield project. Cheers!