I sometimes make a rampage tour through enemy territory, sell all useful buildings and let the cities rebel (or being captured). When the enemy captures the cities, I put a lot of spies into them, so he has no fun in this area. It gets more difficult when the cities rebel towards a faction, but spies are useful here, again.

So, I have a belt of independent cities at my empire's border that delay enemies quite effective. It's a bit like option 2 from your list, just without involving 3rd parties.