I think you have such a solid foundation, that there are a BUNCH of things that you can add that would make the game have a ton more depth without tooo much effort. The one thing that immediately comes to mind that I keep coming back to is adding money, and making the player buy bullets in between rounds. Adding another resource would force the player to have to factor their economy into every decision they make. Do you stock up on cheaper yet weaker bullets towards the beginning? Or do you save up and get a couple of really strong bullets? Do you get something to deal with the armored enemies or do you thug it out and save up? I think it would be a cool risk/reward scenario for the player to be able to call in the next set of enemies early, in exchange for some extra cash. I think the greed mode from the binding of Isaac might be a cool reference for something like this. As it stands, I think it’s very easy to find a combination of bullets that combos well and allows you to go to town on most enemies, so I think making the player work for that would do wonders to make the player want to try again and experiment with different strategies.
of course there are other things too! Such as bosses/mini bosses to keep the player on their toes, more bullets with different kinds of effects and more enemies to use them on!
Got a little long winded there, but I hope any of this was helpful! I think you have a great foundation here that’s just BEGGING to be built upon. I’m more than happy to bounce ideas off you if you ever need!!!