Because all of the actions are terminal, there aren't too many synergies to take advantage of. I prefer going after Provinces; none of the Gardens enablers are strong enough to compete. My preferred build off the 3/4 is to use one Amulet and three Bonfires to get to a 5-card hand*, then build up with Treasure Trove, Bandit, and Gold, in that order. From there (starting turn 9 or 10), Province or Duchy. Usually Witch is dominant, but in my tests against the AI on dominion.games, Bandit seemed to perform better, surprisingly. This strategy can reach 8 Provinces decently quickly (~turn 17-21), so I don't think a Gardens approach has a chance.
*To get to a 5-card hand, open Amulet + Bonfire, and plan to Bonfire two more times over the next three turns. However, you don't want to Bonfire a third time until you have a Silver (either purchased or gained via Amulet). Amulet should usually trash Estates, but look for opportunities to give +coin for Bonfire #2, or Silver to enable Bonfire #3. With best case draws, I can get to a functional 5-card hand on turn 5 (Amulet, Silver, Copper, Estate x2). From there, you can buy Treasure Trove on turn 6 and Bandit on turn 7. The usual case is one turn slower. Use Amulet to clean up your last Copper. Once I start greening, I use Amulet for coin to hit $8 or Silver otherwise.