Yeah, I fully agree with you! The frame drops get really problematic as more enemies spawn. I hadn't thought of pooling, but it is a very good idea! At least for spawning enemies that can work very well. For the ground we are using a tilemap, but I dont know if you can do object pooling on a tilemap. Something to look into though. Thank you for the suggestion!
The spawning actually works as follows: a spot is chosen at a random x, at a fixed height above or below the player. If this spot is underground, spawn an digger, if it is above ground, spawn a flyer. This method should not spawn flyers if there is no place for them to spawn.
And you are completely right about the options in the main menu. We ran out of time for that :/
Thanks a lot for your amazing feedback! I truely appreciate it :)