You are absolutely correct about the Fifth Amendment.
It surprised me when former President Obama suggested, then championed, the use of the “no-fly” list to prohibit firearm sales and possession. Obama is an expert on the Constitution (just ask him) and has taught constitutional law. The Fifth Amendment should hold no mysteries for him, even though it escapes a lot of people like those who clamor for red flag laws.
The use of the no-fly list as a presumptive prohibition would have been declared unconstitutional as it’s a clear violation. It’s quite possible that a lawsuit would be filed as soon as the government or the President tried to put that into effect and that a temporary injunction would be issued.
I keep hoping that someone will file a lawsuit over red flags laws, so those can be declared unconstitutional.