What I would need is a statically typed language that has first class primitives for working with untyped data ergonomically.

I do want to be able to write a dynamically typed function or subsystem during the development phase, and „harden” with types once I’m sure I got the structure down.

But the dynamic system should fit well into the language, and I should be able to easily and safely deal with untyped values and convert them to typed ones.