[pjc50]

But by your definition, you maximise sovereignty by declaring yourself entirely independent. That makes no sense.

No, that makes a lot of sense and accords with most people's intuitive understanding of the word. The trouble is, as you say:

If you are wholly unwilling to enter into agreements with other people then you will find it very difficult to achieve anything at all

And economic 'independence' is a real problem if you want to have a non-agrarian society. You need to import oil, electronics, pharmaceuticals. This is what goes wrong in places like Venezuela and Greece when supplies of hard currency start running low.