"You can't simultaneously support red flag laws and the Constitution."

Sure you can! A right is only a right insofar as it does not infringe upon the rights of others. The public has a right to safety; Allowing a mentally challenged person to carry a firearm infringes upon the public's right to safety; therefore invalidating the rights of the mentally challenged to carry the weapon must take priority.

A person may indeed have been grated a certain right under the constitution, but that right must be conditional upon the mental capacity of the one to whom the right has been bestowed. If that person's mental capacity or behavior changes to the extent that the public is in any danger, there much be a mechanism in place to remove, or at least limit that right. The Red Flag Law, while perhaps not perfect, is a good start in that direction.

Yes, canons also existed at the time of drafting the constitution, but people did not have the ability to carry canons around with them.