I love tower defense. In fact the first game I ever put on itch.io was a Tower Defense game.
Nice twist with the tanks disappearing this seemed to be time based. Would be really interesting to make it ammo based. So then you could have an expensive but super powerful, 1 or 2 shot towers that you put right at the end of the path. While you loaded up with cheap but weak and lots of ammo stuff at the beginning. Might create a new tower defense dynamic.