I don't know Godot, but you should apply some rules for off-screen enemies, like: "Don't render, don't move, don't have AI". Also you should increase the radius of spawning and add a minimum spawning range. I strongly recommend to look what other hack & slay games (PoE, Titan Quest, even Diablo 2) have in their main UI, how they explain stuff in the main UI and basically copy stuff until you can add your own flavor to it.