The Laravel developers expect you to use Stringable::macro if you want to extend their 'fluent' API, and if you're working in an organisation that builds with the Laravel toolchain that's probably what's least surprising to the other members.

I'm still not clear over what you want to do or why the third party library expects there to be a hacked in 'fluent' method on the Stringable.

Personally I'm not a fan of the Str/Stringable API:s, I find it weird and confusing to mix methods for file paths, strings, encryption and so on. Actually, I'm more of a Symfony person for reasons like this.