So, why do they move like boats? Why can't my ships spin 180 degrees and fire while moving backwards during a retreat?
That is a good point. In normal space, you would fire your engines to move you in one direction, then turn them off. Then, you would fire your thrusters to reorient your craft and open fire while moving in the opposite direction. While firing, you would increase your rate of speed in the opposite direction. Either that, or you should have some form of rear-facing turrets to fire while retreating. Rear-facing turrets would make more sense since you could continue to use your main engines to accelerate away from the enemy.
As for Star Trek-like battles, I would not expect larger ships to be able to do that. Stargate SG1 and Stargate Atlantis both show ships moving how I would expect them to move. Large ships stay relatively motionless or move mostly in one direction or take time circling even larger, less mobile ships unless the larger ships have such vastly better technology that they are more mobile. The smaller craft like bombers and fighters have the greatest manueverability. But, even in Stargate, the ships do not sit still. They are constantly in motion, even the capital ships (unless it is a capital ship and its fighters against only small and more mobile craft). In general, if capital ships are facing other capital ships, they move.