haha, I made a no upgrade run once too! we gotta work on proper balancing still in general
If you are going to balance it, it would be good to promote the use of the mechanics within the gameplay:
Maybe when you pick up a tower it can take the direction of the nearest one and the tower it points to creates a sprite of the enhanced tower above it with low opacity. A beam of electricity pointing towards the tower to be joined can also work!
Success with your game! I will be attentive if you update it