To start by speaking well, the art is generally quite nice. The sfx are good, the music is fine.
For constructive feedback, there is a game in here, but the gameplay needs an overhaul. The main character needs to start off with some of the upgrades already unlocked so that he's a bit faster and shoots a bit harder. All enemies do a full hit of damage, but healing only goes up half. I think there should be a mix where some enemies do less damage, some do more. It feels like the half healing should be a more for harder settings. It would also be good to have healing between areas.
I would get rid of the cardinal movement in exchange for 8 way movement or free movement, similar with shooting directions. Having to line up with enemies is a pain, I don't feel powerful or in control. I'm just stuck sitting until enemies happen to line up with me, or I manage to line up with them.
For stage design, I would recommend more like Zelda dungeons, where you have a variety of challenges. Kill the enemies, solve a puzzle, fine a key, etc. Having the nonlinear design can make it a bit more immersive. Also having tailored rooms can make the encounters more interesting.
Right now the monsters have no synergy. I only got to the third area, but there's no monsters so far that inflict status effects. No monsters that heal each other, or buff each other. There's no real differentiation between flying or land enemies. If you had water or obstacles, flying enemies might cross them, etc, etc, etc.
For powerups, I'd recommend newer guns, or devices you can use to change up how you approach the game. I can give more feedback, but all of it will pertain to expanding what you have, refining or polishing it, and just adding more.
Anyway, congratulations on making what you have, getting to a finish line. But I strongly recommend a remastered version that takes your foundation and goes all the way.