I really like this game. The main issue I have with it is the timer on the spawn tree button, its too slow. I don't mean in a balance way, but in a playability way.
Most people can get into a rhythm pressing a button, but anything over 2 seconds and it becomes very hard to 'know' instinctively when it will come off cooldown, so you end up just clicking constantly.
A shorter cooldown on it makes it easier to get into the rhythm of just pressing it when it comes of cooldown without needing to stare at it constantly or click all the time.
The other thing would be to add a loading bar type effect over it so you can see when it will be available again.
It's the very first thing anyone who plays the game interacts with so it needs to be 'perfect'.