How to go about spawning waves of enemies and having an upgrade menu?

Hi, I'm building a 2D game in which I spawn a wave of enemies. However the issue I'm having is deciding the best way to go about it. I want an upgrade window to appear after completing each wave. Should I put each wave into it's own scene and then flick between the next level and an upgrade scene. Or should I have 1 game scene and flick between that and the upgrade scene? Nothing changes in terms of the game scene, only the enemies will become more difficult on each wave. Any help is greatly appreciated.