Keep in mind, it’s just as inconvenient for the attacker to make those 30+ jumps three times around.
Alternatively, either side could make the trip once and wait out the timer in the area engaging targets of opportunity.
Or, if either structure is near a decent trade hub, they can just clone jump (or hop in a shuttle) en masse and buy an attacking fleet there.
And what about structures in adjacent systems that happen to be in different regions?
I get where you’re coming from, but it seems like too arbitrary of a restriction with little-to-no benefit.