How can arm repeatability be improved?

I have read the documentation very carefully, I have a milling machine but without ATC and I see that it is an industrial solution to use a robot to use it as an ATC, but maybe it would be good to add some motor to improve the fluidity of the movement (horizontal rotation), but it worries me the precision in the repeatability of the arm to leave the tool in the spindle and exchange it for other tool.

Therefor, what is better??:

1. install encoder to the current solution,

2. change the motors for motors with closed loop.

3. change to servos-driver.

4. change to servos-driver and encoders

Regards.

Reach: 24.75 inches (62.9cm)

Payload: 4.15 lbs (1.9kg)

Accuracy - .01mm

Repeatability - .75mm