If you subscribe to the Lean Startup ethos, then testing your hypotheses should be your primary goal.
> As you consider building your own minimum viable product, let this simple rule suffice: remove any feature, process, or effort that does not contribute directly to the learning you seek.
So the answer here is probably... Whatever you're the most comfortable with! Most of the code/infra you are building in the early stages is likely to be nuked at some point anyways :)
Sticking to your existing toolset also has the benefit of allowing you to get faster over time, as you gain familiarity with your toolkit and build reusable abstractions to reuse in your next pivot.
The Pydantic page itself provides some pretty nice notes on why it's awesome: https://pydantic-docs.helpmanual.io/#rationale