Thanks for the kind words. I actually tried to make it so each character had a low damage/low strain cost attack and a high damage/high strain cost attack. Enemies having the ability to inflict heavy strain is a good idea and would probably be easy to implement.
You are right about the lack of content. I actually put some smaller enemies in the game but it made the game very difficult. I ended up cutting them since I didn't have too much time to balance things.
The combo suggestion is very good. I'm not sure if you've noticed it but the characters can attack even at 100% strain. My initial idea was that at max strain you couldn't act for some time. And the enemies would also follow this rule. I think that'd make it more interesting to plan huge combos. Of course I ended up cutting that feature too.
I will try to streamline the UI design as you suggested.