back to OP:
in general the idea is not bad, but i would not agree with the outpost things.
i would bind walls not to outposts but to thething that we dont have atm. PBS!
so if you have a PBS you can place walls in a certain radius around it. that will help walling off entire ilands with 3km long walls. with PBS comes costs and maybe a special module that prevents the decay or baybe a slow self repair for the walls.
an additional feature that may be worth a thought (even if it makes no sense from the logical point of view) is to limit the number of connected wall segments to a fix number (not saying what the number should be). a wall only works when it closes the gap you want to be closed. if there is one hole then the wall is for nothing. so if you limit the total lenth of a wall your option for spamming will be limided, because you can not wall off too large areas on plain field. you can wall off on smaller gaps between mountains, or on strategic locations, but it will have its limits where and how it can be used. but that is just a thought.