Thanks for playing! Glad the controls weren't too tough (and also that I'm not totally crazy regarding the difficulty lol). Overall (especially with health and invincibility after getting hit) the game's fairly forgiving.
I was able to beat the game on the hardest difficulty on my first try, but yeah without more enemy types it's hard to have it be properly tough without being just unfair. I mostly prioritized features other than new enemies because I was more interested in coding waves and an upgrade system and such.
The shotgun deals a little more damage, but is mostly useful because it can hit any number of zombies in range at once, and pushes them back to give you space if you get overwhelmed.