Don't depend on documents to defend your individual liberty. The US Constitution is a joke. The government of the US certainly doesn't follow it at all. An amendment to it wouldn't help either.
The key is whether an agreement is voluntarily entered into by all parties. Can I leave that agreement without violence or theft being used against me in the future too?
"Can I leave that agreement without violence or theft being used against me in the future too?"
Yes, according to the Panarchist metapolitical theory everyone should have the freedom to voluntarily enter and exit any government without having to move to a different geographical location.
To enter a government you would simply sign the explicit social contract and agree to the terms and conditions of the contract. It is conceivable that Governance Service Provider contracts would act someone like the contract you sign with your phone company. There could be 1-Year contracts, 2-Year contracts, and potentially LifeTime contracts with conditions and fees for breaking the contract, but I doubt LifeTime contracts would be very popular.
Either way, as you say, "The key is whether an agreement is voluntarily entered into by all parties."