It's a tough answer because we have had years of artificially-pumped support and development and ecosystem growth of Pydantic.

But if I had to roll the clock back I'd recommend marshmallow and that entire ecosystem. It's definitely way less bloated than Pydantic currently, and only lacks some features. Beyond that, just use plain-old dataclasses.

https://marshmallow.readthedocs.io/en/latest/