Freedom in Space
If frontier societies are not always free we cannot assume that Belters or Spacers (or similar future societies in Deep Space) will be free. I suspect that rugged territory that isn't amenable to military action might be the best place to establish a free society. What would be the equivalent of rugged territory in Deep Space?
My guess is that dwarf planets might be the best place. A settlement on or in a very small asteroid might be vulnerable to a laser or bombing attack. A settlement on or in a dwarf planet can burrow inside. On the other hand, a settlement on a large planet (e.g., Earth) cannot cannot burrow that deeply against the pressure gradient. Is there a sweet spot?
On the other hand, maybe a three-dimensional society will suffice.