Nice, thanks so much for the feedbacks!
I actually made the enemy stats a bit lower compared to the player's to make it easier, but if it felt too easy, I'll try tweaking it again.
And no, the SFX are also from OpenGameArt. I actually forgot to put them, so I'll update the game page later, thanks for the reminder.
All of your suggestions does make a lot of sense, especially unit progression system. I have some idea about it, but nothing concrete yet, and the speed up button is definitely a good option to have.