I found the optimal strategy is to just spam farms until the map is filled with them and you have enough money to place a few more when it rains and turns 4s into 3s to place more on. Then pick a tile at 5 elevation and constantly plant trees on it for the rest of the game, filling in farms on every tile that becomes available. Once there's no more space for farms you spam the spacebar and plant trees on the last tile for the rest of the game.
If I had to change something, I would add a score value that increases by the total remaining land in the game including elevation. That way you're encouraged to hold onto multiple tiles instead of just one. Also more buildings with special effects that can affect neighboring tiles or move clouds