[Libertarianize the GOP]

I say the only way secession is legal is by way of a Constitutional amendment.

You are entitled to an opinion but would you care to elaborate on why. Article IV section 3 provides for the Addition of new States, if power can be extended over new people and territories then it likewise can be rescinded to allow a Voluntary mutually agreed upon secession. It also provides for the US to dispose of territories or property. If all sides can’t reach an agreement then any provision in the Constitution would be meaningless.

[Dixie republican]

if power can be extended over new people and territories then it likewise can be rescinded to allow a Voluntary mutually agreed upon secession.

If that were true, why didn't the Founders say so? They did a pretty good job enumerating what they wanted. You don't think they even considered secession?