In the Pydantic documentation, the example for customizing the schema completely with the __get_pydantic_core_schema function override uses core_schema.no_info_plain_validator_function.

    def __get_pydantic_core_schema__(
        self, source: type[Any], handler: GetCoreSchemaHandler
    ) -> core_schema.CoreSchema:
        ...
        return core_schema.no_info_plain_validator_function(validate)

In your code:

    @classmethod
    def __get_pydantic_core_schema__(cls, source_type: Any, handler):

        cls._schema = _CustomModel._make_schema()

        fn = core_schema.no_info_plain_validator_function(
            function=cls._validate,
            schema=cls._schema,
        )

        return fn