Hi, and thanks! It’s definitely very challenging. We do intend to continue developing it, perhaps once Mini Jam 111 is over.
The game doesn’t have any objective upgrades, but hitting enemies and absorbing shots with your shield will charge the bar in the bottom-right corner. When that starts flashing, you can right click to use its “special shot”. Killing some enemies with the special shot allows you to pick up their weapons, which have different behaviours. I suppose it’s more of a horizontal-grade than an upgrade