Weapon fire rate for me as the first thing to max. But that's a fair point, and as part of the difficulty in the waves I was making them able to spawn further apart.
Yeah unfortunately that does become the best way of getting over the hurdle of level 5(ish). I didn't get the balance right in this one, but I felt I could at least ensure people had the chance to make it through :), by sending them back in with 8 lives and all their upgrades. No one plays Jam games for longevity :P.
Hope you had fun with the story elements.
Thanks for playing!