I think you lost track of how valuable the different cards are when the opponent has the thief.
- Copper is still an obstacle to your deck but the opponent will steal some it from you, so on turn 5 you could have not trashed the copper and used the extra coin to buy a governor.
- Silver isn't a great stepping stone here. You probably won't collect enough to buy provinces, it might get stolen, and your opponent might give you some as gifts anyway. Again when your opponent played a governor you could have trashed estates for villages or wishing wells instead of silver. Villages allow you to play with moats and even an adventurer who might find a niche in this kingdom.
- Gold is still good but your governors can give you more gold as and when you need it. The rest of your deck development should be focused on making a nice engine to that can use the gold effectively, especially when your opponent is likely to trash your estates and copper.