In fact, I am a TD novice. After playing a lot of games, I finally realized that placing the tower in a certain place is the best...
2000 beans really left me at a loss... So I decided to gamble. Actually, there's a trick to gambling, and basically you can get what you want. It's half the price, but I'm not sure if this counts as a bug.
In my opinion, in the early stage, there's no need to combine beans or build towers. Try to combine weapons instead. In the first half of the game, skilled players don't need to place the ultimate tower. But I did (I'm a noob).
One more thing: try not to use the big plan. It feels great to see the boss slowly tortured to death by yourself.