I think your game is very cool, criminally underrated and for a solo dev, it's very impressive ! I think it's a mix between doom (the og) and vampire survivors so I'll compare your game to them, but I do understand that you worked alone for 4 days ; I gave your game a high grade, it's more if you want to keep working on your game :)
In doom, the best part of the game is not really the combat, it's the freedom of movement and how fast paced you move. the level design is also a big part of the appeal (not the button pressing however, but the verticality) so having obstacles would have complemented the shooting a lot :)
In vampire survivor, a lot of the fun is from the "high risk/reward" that comes from collecting xp. It probably would have be better to have the enemy drop the token; or some reward to make risk taking more appealing. As it stands, its a really cool game with avenue to improves.
I saw that you wanted to add a tutorial, but I don't think it's needed. I do agree that it is sometime confusing, so here's something you can do to make it less confusing :
- for the eye, I didn't understand as well what they did. Adding a clock to their sprite, with arms that rotate fast around the center of the eye, would probably be very effective way to tell the player what the eyes are doing.
- for the shop, give the player a few seconds at the end of the wave with something like "wave defeated, opening shop in 3... 2... 1..."