How to set the cutting of 15mm and 18mm plywood?

Try a test cut at speeds 5 - 15 MM/Sec at 85-90% pwr

If your focal length is short (2 inches or at worst 1.5 inches), it will be very difficult or even impossible, as there is no way to obtain a sufficiently narrow 'burning point' across the entire height of the material.
By using a focal length of 2.5 inches or 4 inches, you can achieve results, but you must slow down and have a powerful air assist to blow away dust on the cutting path.

Plywood can be composed of almost any combination of wood/adhesive layers. Some are easy to cut, some are almost impossible