You are proposing that anyone should be able to simply associate with who they want and then, with those people, form harmonious societies/countries/states. Am I misunderstanding this?

No that is correct.

Can I, as a middle school kid, just… go to another society before I’m able to explore who I am?

This is my own opinion, but children (the specific age we're talking about depends on the subject and the culture) aren't generally competent enough to make important choices like moving to another country and therefore it's usually in the child's best interest to have their parents determine things for them like where they live. It also seems part of the natural order that the parents/guardians of a child get to determine how they are raised, including what moral values they are taught. While I may not agree with someone teaching their child to be Muslim, who am I to deprive them of the right to raise their children according to their values?

Some societies might decide to give children rights to autonomy, and I'm inclined to think that a society should be allowed to try that, although it will end in disaster sooner or later and other countries might in my opinion have a moral obligation to step in eventually.

But you are proposing a more “just” starting point (inside each society) which, within a generation, may be almost indistinguishable from our current situation. How does the previous generation having consented make it a more legitimate government for the following generation?

No I'm not proposing a starting point. I'm proposing that at any time someone should be able to decide they don't like the country they're in and either move to another country that will accept them or take some land and secede to form a new country or join to an existing one that will accept them. So every competent person once they reach adulthood (however that is defined by the country of residence) will have the opportunity to leave their country, or do so later in life. This means at any point in time the country is comprised of people (and their children and mentally incompetent dependents) who consent to be under that system. That's what makes the government legitimate at all times and not just for the first generation.

Unfortunately, we are stuck trying to collectively figure it out something that we hope works for as many people as possible

But why are we stuck in this situation? Why couldn't we have people separate into groups that agree on things like they naturally want to? Why are we forced to make some compromise between the group A that wants X to be illegal and group B that wants X to be legal? And then either group A is forced to violate their consciences supporting and paying taxes for what they consider evil or group B is punished for what they think is fine or even a moral duty? That just makes groups A and B constantly fight each other and form cliques and the country is divided and inefficient. Why? I think the reason is obvious. Because those who rule the country do not want to give up their power, most of which they would do by allowing secession. But things don't need to be this way. With enough public recognition of the right to individual secession it would have to be allowed. And once one country allows it many will want to move there and other countries would be pressured to uphold the right.

I still don’t think it’s good to set the precedent of “If I don’t like what the government is doing I don’t have to pay my taxes.”

But how do can you deny the argument that taxation is theft to those who never consented to their government? It's just taking people's money under threat of force and using it however the illegitimate government wishes. Even if the money was only used for the most upright causes with maximal efficiency it would still be theft because it lacks consent. Charities can't take my money unless I made an agreement with them, and in the same way governments can't unless I made an agreement without coercion.