In principle, I think this works very well. The idea is solid, it feels crunchy and responsive. My only gripe is that, in practise, having more ships as the invader does not feel like an advantage at all. With fewer ships, there's just less for the defender to hit and more consistency for the player. I found it felt easier when I had just one or two ships left and I could reasonably dodge the defender's bullets.
So, maybe needs some polish in terms of the core mechanic, but broadly a very fun game!