How thick should a topper 180x200 be?

That depends on what you want to achieve. A 4 cm topper, like the Tuur® Mono Topper, adds a soft comfort layer without fundamentally changing the feel of your mattress. An 8 cm topper, like the Tuur® Original Topper, noticeably changes the sleeping comfort and also adds ergonomic support. Do you have back problems or sleep on your side? Then choose 8 cm. Just want a softer top layer? Then 4 cm is sufficient.