If any permanent has an ability that triggers when the permanent you return leaves the battlefield, that ability will be put on the stack after you finish casting the spell, and it will resolve before that spell
Only adding counters after saddling weakens the card dramatically
It also pairs well with discard outlets in The Gitrog Monster decks where it can go infinite
If a third landmark counter is put on Treasure Map by something other than the resolution of its ability (as modified by any applicable replacement effects), you won't remove those counters, transform Treasure Map, or get Treasures yet
This card feels like an excellent finisher for mono-green decks because it can come down early, survive certain types of targeted removal and board wipes, and then buff up your entire team to a game-ending threat for five mana
In a multiplayer game, it does expire if you leave the game