Unless mutability is needed internally, this would improve Datatypes on multiples areas, and would be a first step to a more in-depth future improvement of datatypes API.

1. better performance and memory efficiency of tuples vs list
2. possibility to type hint them more precisely. Since a lot of children types are of static length (list, decimal for example), tuples are just plain better than list to statically type.
3. Later, the "know length" children could be returned as NamedTuples (if that's possible from the C++ side, I have experience with Pyo3 but not much with C++ ), improving the user experience.

I have already code that exist for this and would be happy to make a PR for the static typing side.
Also side note:
would be nice if JSON and BIGINT could be exposed as constant just like the other "simple" types.
https://github.com/OutSquareCapital/pql/blob/7a9f8ec00c55cd8d8c0eadd7dfb85ad6d1aa1dcc/src/pql/sql/_datatypes.py#L63