Consider, though, Article I, Section 8 “The Congress shall have the Power To: ... make Rules for the Government and Regulation of the land and naval forces” clause. I’m not so sure the “unconstitutionality” of the War Powers Act is so cut-and-dried. The Founders were opposed to standing armies and certainly to the Chief Executive as King. The Continental Congress wasn’t hands-off when it came to the Revolution. I think a framer’s analysis of the WPA would find it to be a very good balancing of powers while permitting the CiC to act in extremis in defense of the Nation.
The Act was written purposefully to skirt around the Constitution - they knew what they were doing.
But any rational analysis says that the President cannot start a war (i.e. initiate hostilities against a legitimate sovereign government) without Congress declaring it, and that he cannot have the resources to conduct any military action at all unless Congress signs the check.
However, once Congress signs the check to fund the armed forces, he is the Commander in Chief of those armed forces and can deploy them in any way he sees it necessary to execute his legal responsibilities - for example enforcing UN resolutions.
The WPA is an attempt by Congress to keep their powers while taking away some of the President's.