Thank you for this feedback, this sort of detail is really useful for me to improve the game. The HP drain over time is actually to add an alternate damage source to the player that isn't dependent on the enemy spawning, which mitigates the effect of random spawns or missed shots in the player's progress. But I understand its a frustrating mechanic and I would like to improve or replace it eventually.
I'll keep everything you've said in mind as I work on more updates, and I'll try to make the UI clearer. Thank you :)