Incredible work on capturing that PSX Era! Level and Environment design was amazing and on point! Gameplay was fun as well!
However, I would definitely look into adding in;
1) Different Weapons
2) Reloading and Ammo Management
3) Power ups
Then you can actually take this in 2x Directions; As a wave spawning mode, where every wave increases Enemy spawn rate and count.
OR (Which I really think you'd thrive with this)
An Arcade On Rails Shooter mode
I really believe with your skills in level and environment design, you d be able to make a great on Rails experience!
Then some extra polish; I d suggest adding in a custom recticle, muzzle flashes and more impact vfx.
A Solid experience! Well done!