Yeah so I do like these suggestions, they keypress thing to buy upgrades is such a simple yet worthwhile addition. I'll likely work on this a bit more once the voting period ends for the kenney jam.
As for the immunity for a temporary time, there is a short immunity frame of about 0.2 seconds (Your character turns yellow) upon getting hit but I can see how that might be a bit too short with the amount of enemies that can pile up. Thanks for the feedback though!