Nice game! I really like the combination of shooter + tower defense! You could go even crazier and add RTS elements with controllable guards with various abilities!
RE the theme: I think you're too hard on yourself, MANY submissions just used the concept of death -> restart with slight changes to gameplay to represent the theme and I feel like your game does that as well and even better than others, so I'm gonna give you some theme points for encouragement even if you're begging us for none! :D
It just needs some epic music!