I did not! And yeah, just some tutorial messages in the background would have helped out a lot to explain all this stuff. My first experience was 'mess about figuring out the controls, discover weapon-switching, find the pew-pew gun, stick with pew-pew gun because it Shoot Good better than the others.
Free-shooting + pixel art is hardly impossible: you can do it with 6 frames (up, up45, forward, down45, down), flip for the opposite direction and as long as the bullets are free-aiming people generally won't care if the character isn't (though you'll need a crosshair for obvious reasons). You'd probably need to split the sprite into upperbody/lowerbody if you want to run-and-gun and still stay sane as an animator, mind.
I wouldn't say you need complex AI, just to build your levels around the AI you've got. As it is, there's a lot of open space, so the player can just outrange the goons, and the tank is only threatening when you can't casually jump over its shots. If you want players to be dodging attacks at close distance... force them to close the distance. Even giving the mooks a range boost would help as long as they can't fire from offscreen.