Basically, anything beyond day 20 is not very fun anymore because there are too many enemies.
This forces you to make a choice:
1. Use a gun that hits multiple enemies at a time, usually something overpowered like Gravity Gun or a very strong Bowgun.
2. Use any other gun and have a very difficult time winning due to the huge number of enemies.
The solution? Stronger enemies in fewer numbers. This will also help with the fact that allies are USELESS when there are many enemies with small amount of health on the map, because they will take as much time to aim at them as they would a stronger enemy, but by the time they reach it you probably have already killed it and moved on to the next enemy. Allies help most against high HP enemies, which there aren't enough of even by day 30.
Also, there are too many high HP enemies that fire at you from too far away (catapult and dragon can shoot from just the inch from the far left side, may be a bug and only happens sometimes... might have something to do with knockback) and chickens can sometimes even shoot from OFF THE MAP, which is very annoying. Chickens have far too long range. All enemies should have to come to at least the center of the map to fire at the player, I think.
Single-enemy shooting guns need more damage and auto-fire guns need more accuracy. That would make them more useful.
If you give all of your allies gravity guns the game will never be difficult anymore. The solution is to make gravity guns have a smaller area of damage and move faster across the map (to prevent them from hanging around and killing tons of new enemies).
Also, "luck" has less to do with what items you get and more to do with what waves of enemies arrive. I would like to see more "luck" come to play with items and town events.
OTHERWISE I love this game and I am not expecting it to become more complicated or anything, I think it is a well-made little coffee break game. Well done Noa!