Regarding the upgrade tree, yes, I usually make games where you get items by defeating enemies and can use them immediately. In other game genres, the upgrade tree requires a lot of thought and careful consideration, unlike incremental games.
The boost option is just a bonus because I didn't have time to properly balance the upgrades (not enough time to test). I apologize for this. The same goes for the music (using old songs I had saved).
Most importantly, thank you for the suggestions. I will try to balance the upgrades better and make the gameplay faster.
(I've now added A* pathfinding. The heroes look much more lifelike.)