Interesting concept. This feels like it would be a great mechanic for a roguelite.
I just wish there was a reason for me to stop holding LMB. Since some time into the game I found out that I don't actually release my finger from LMB. Not trying to say that this is bad, but , in my opinion, adding a purpose to stop firing will give the players a room for tactical thinking.
However, I still had fun playing your game, which is more important. Great job!