OK.A gimbaled mount is under your direct control. It has a bit of aim-assist to it, to keep you on target.
A turret fires on its own, once you start it up (or completely on it's own if you set it that way, which I don't on the 'vette).
A fixed mount has no aim-assist (well, a very tiny amount), but puts out higher damage. But you gotta be able to aim well, which I really can't
I'm not sure why you're having problems killing stuff, though. Maybe you'd do better with kinetic weapons once the shields go down, I don't know. I don't care for beams, but most people do. I'm not exactly an expert, I just know what works for me.
I think I'll plagiarize your build and give it a go and see what happens.