Interesting take on pong. You do however have some major bugs and I'll provide a step-by-step so hopefully you can reproduce it. After failing mission 1-1, I bought another ship, failed mission 1-1 with ship no.1 and 2 destroyed. Started mission 1-1 once more, alt tabbed after hitting a key to start the level and since your game does requires focus to continue playing I alt tabbed right back in, nothing happened so far so I let the level run until my planet got destroyed and I got some cash. Re-did the level with the same procedures, hit alt-enter to go into window mode, alt tabbed as a test to make sure the application really needed focus and alt-tabbed plus alt-enter to return to fullscreen. After that was done, alien ship spawns never managed to enter the game space(always off-screen) and they self-destructed, allowing me to skip several sub levels(from 1-1 to 1-5) by just opening up a level, waiting for around 5 seconds and getting a mission complete. Add some description text to menu modes such a mission mode, their icons don't really tell me anything about them. Here's a small webm as an example of it happening.
https://my.mixtape.moe/bsnkcx.webm
Game seems fun so far, just gotta go through some necessary upgrades in order to bring in more cash than I spend repairing/buying anew. Oh also, allow the player to collect the cash(or give whatever is on the field and will drop from an alien ship) before you transition to the level complete menu. Good luck.