Well done putting together a fun game! I'm a big fan of Plants vs Zombies, so this is right up my alley. I had trouble with things lagging when they spawned. I noticed @aughckt's suggestion about preload()... there's also a page in Godot's documentation about other ways to fix lag: https://docs.godotengine.org/en/stable/tutorials/rendering/jitter_stutter.html
Overall I enjoyed the game, keep up the good work!