I’ve just come across your post and I make a sketchy preliminary response below:
I think the problem may be that there is a fundamental vagueness about what you mean by forming a voluntary democratic government.
You are free to join together with others in an association and set up some kind of joint authority if you so choose You would be foolish to do so, however, without a clear and firm grasp of the terms and conditions on which you agree to be governed, including what you can do if you are oppressed. Among those terms and conditions that you should insist on having in your agreement would be some which would outline what should happen if you decide to withdraw from the association.
Let me sketch two different scenarios. In the first, before you join the association, you own your own property, and the exit agreement would be such that if and when you exited, you retain your property. When you do exit, you are no longer subject to the authority of the association. In the second, you might (as in the utopian associations of the 19th century) set up a community on jointly purchased land with others in such a way that only those who subscribe to the authority of the community could continue to live in it. In such a case, the exit condition should include some measure by which the government of the community purchases your property from you at a prior agreed price.
In neither case, can the actions of others subject you to the authority of an association.
In our present case, you cannot withdraw from the state and continue to live on your property, associating with others or not as you subsequently choose to do, as in the first scenario. The state operates as if you had voluntarily committed to a joint venture (as in the second scenario) where withdrawal requires an exit from the territory of the state. But most of us have never made any such voluntary agreement with the state.