About the enemies: that's interesting that you were considering cutting out the shield enemies. I definitely think it was for the better you kept them in, in the end. They were the enemies that forced me to move the most and to actually plan around doing stuff. Melee enemies, I just had to back up while shooting at 'em. Ranged enemies, just moved side to side while shooting. It was the shield enemies that ultimately forced me to move around, especially then when they were blocking the shots for the other enemies. You made do with the time you had and I think it worked out well.
And oooh, about looking to enhance the spawn system... that implies you would be working on this game still. If so, I'd definitely be happy to give it another go when that time comes.