I love a game that makes you work for upgrades! Balancing resources was tricky but just added to the challenge. Got to wave 6 on my first run, was trying to hoard cash for the upgrades, realised I probably need to spend more!
EDIT: Came back to play again, managed to get the final ship upgrade, fill out every slot on that ship, and managed to make it to wave 18! If you ever do more work on this, please let me know!