Well, the inflow of credits is relatively constant, you could tune it relatively easily, or add sinks. E.g. have repairing consume credits without a "repair all" card, and more limited or niche repair/upgrade cards
After that, to limit frustration, perhaps you could switch turrets over to consuming ammo rather than passively decaying, with some kinda faux system, like... say, each next shot within a wave uses less ammo, so that turrets are unlikely to run out within one wave, and almost guaranteed to run out in a few of shooting at all.
For interactivity, I think it'd be cool if turrets were on turret slots, with the ability to replace individual ones using cards - maybe even add new slots, or give tiles abilities like "pay 1 to switch lane direction", "pay 2 to erect a destructible blockade", that you can add/replace with one-time cards, so you develop your tiles over time(ones that go to stack wouldn't regenerate their turrets, here, I think)
Could also be cool to have a bigger map, since currently horizontal tiles don't have much of an use.