Time-based temporary upgrades feel kind of irrelevant when the strategy is to take it so slowly. Like the temporary shield that is in right now maybe encourages you to go in and shoot a lot while it lasts, but then the ricochets are still around afterwards. But a ricochet shield could maybe last a number of hits instead of a number of seconds. And a weapon could last a number of shots.
If it's puzzle-like, you don't strictly need enemies. The goal could simply be something like shooting a tunnel through to a destination, while surviving your own ricochets that last indefinitely, or something. Not saying that is better but something to think about. Kind of a twist on Breakout.