help with coding a stepper motor triggered by a PIR motion sensor

That would give you a resolution of just over 2° which is a lot better for control. The servo will try to move to the new setpoint as fast as possible, which at 1° steps can make for a reasonably smooth slow sweep but at 18° steps certainly not.

Anyway do read the spec sheet of that 6-turn servo carefully as it may work a bit different there.