Y’all, this isn’t even close to hard. Congress created the IRS under its Article I, Section 8 constitutional authority. Of course it has oversight authority over the agency and the taxes and returns it collects. The executive branch only has the IRS and its returns because congress passed the laws that brought them into existence. And more to the point, since tax returns are only subject to privacy protections because of the will of congress, the idea that there’s some right to privacy beyond which congress cannot legally reach is absurd.