Ahhh, yup. I did not realize I could level the runes. I did increase my mana regen rate, and I tried pairing that with dealing 20% (I think) more damage, and then with taking 10% less damage. I bet leveling the runes would have gotten me there. I often miss chances to min/max a few times before it really clicks for me, but then I can get good at it.
Music is great for setting the tone and feeling you want the players to experience for sure. Like how intense do you want it compared to if it should feel less intense and more whimsical, etc. The audio feedback you get when you click on a spell, or when you launch a spell can be very satisfying too. Now that I know you can level your runes, you can imagine that if someone invests in a rune, you can reward them with the sfx that play when you cast it being bigger and cooler, for example. In my opinion, it's those kind of thoughtful touches that help us players really feel something when we play. Also, it can confirm things like when you take a spell out of the queue - there's audible confirmation with an appropriate sound that you did it. That combined with visually seeing it helps it all come together and be cohesive.
I could talk shop all day long. I love it. Reach out anytime! You're doing great!