Very surreal game, certainly original! Once you get the concept it is not too hard, and the gameplay loop is a little repetitive. I'm guilty of not explaining mechanics in game too - it's easy to skip in a Jam but always super valuable for the inevitably confused players.
I'm not sure exactly what I'd change except add a little spice and variety. Instead of engaging with the other mechanics I just fed the beast, unlocked all turrets, then upgraded them, and won.
Maybe the entity possessing the mech has a mental link with you gives you a warning about the highest priority approaching threat every few waves, like a shoot a missile at a super dangerous tank approaching from the west, or they're throwing all their enemies at us this wave and use the big bomb to kill them all. Maybe add a passive income source too? Just a few ideas. Great job pulling off getting so much content done in a game jam.