What about

def validate_option_b_input(value: Optional[list[float]]):
    if value is not None and len(value) != 2:
        raise ValueError("Length must be 2")
    return value

class OptionB(BaseModel):
    min: Optional[float] = None
    max: Optional[float] = None
    value: Annotated[
        Optional[list[float]],
        AfterValidator(some_eventual_validation),
        AfterValidator(validate_option_b_input),
        Field(validate_default=True),
    ] = None