Many serve problems actually start with the toss. If the ball goes somewhere different each time, the rest of the motion has to adjust, and errors follow.
A consistent toss location allows a repeatable swing
Toss height varies between players; what matters most is that it fits the timing of your motion
Releasing the ball with a straight arm and an open hand reduces spin and wobble
For different serves, many players change the toss slightly, but too much variation can tip off the returner
A drill I often use: toss without hitting and let the ball fall. It should land in about the same spot each time, slightly in front and to the side of the front foot for a flat or slice serve. If it lands all over the place, fix the toss before working on the swing.