I appreciate the feedback, you are highly perceptive. Many of the points you mentioned are already on my to-do list, I just didn't have time to fix them in time.
However, others such as the perks should work as intended in the Windows build. If you downloaded the Linux one then that would mean 100% of the people who did have contacted me to tell me things don't work which is rather tragic. That version is no longer available for the time being.
Without perks I can believe leveling up feels too slow because they make up a large portion of the player's strength. You almost made it to the end of the demo anyways if you reached a desert battle in stage 5.
Finally, I must inform you that Rebel Angels looking and sounding like insects to varying degrees is intended, and shall persist.