I'm so glad you loved it! Metroid and Super Metroid were a big inspiration for it. I'm glad you liked the boss, too. I spent many days trying to get that battle in a good spot.
The one HP boost might be a bit too hidden, haha. I was thinking about having the map rooms change color based on if there's an item in them like Resident Evil, but never got around to it. It's earlier on in the game, before entering Sector A, but you need the wall jump to reach it.