Don't want to say that you played perfectly, but maybe this game is also one where you can blame it on bad luck a bit. So there basically are no cards at $5 that you want, $4 is difficult.
Your opponents/you:
T3: $6/$4 Gold/Silver huge difference
T4: $3/$5 WW/Royal Seal. With the Gold in his deck, he can savely go for Cantrips already, which will be much better later than treasure with the KCs. You are "stuck" at $5, with nothing you really want. RS>Silver, but it's just another treasure that can't be KCed.
T5: $4/$6 WW/Gold. There finally is your Gold
T6: $7/$4 GM/NC.
T7: $7/$5 KC/RS
So could you have done something better? Probably. I wouldn't have bought the second RS for sure (as you have your Gold, maybe fall back to WW also already), opening NC vs. Monument is just a small difference for this start, and I really don't really think it's a mistake. You want your buys, even Woodcutters, to be in hand fast and not miss the reshuffle. The one points from early Monument plays does not matter much if you win the KC- and GM-split
But the variance of the draws is just higher for you opponent, and with nothing really good for $4 and $5 the early Gold is just huge, allowing him to take WWs for $3 without worrying for his economy, and getting the Cantrips to collide with the KCs later.