Is it outside? Several things can cause cupping, but the usual cause is an uneven change in moisture content. If one side is wetter or dryer than the other, any board will cup. 200mm is a fairly wide board. Using quartersawn wood will usually result in a more stable product, but quartersawn boards are typically more narrow, and may ned to be edge glued to get the required width. Sometimes cutting your 200mm board into narrow strips and gluing them back together will make the board more stable.
Sounds like the spitting is also caused by exposure to the elements.
If it is outside, you need a much better finish to minimize moisture content changes. The best is 2-3 coats of epoxy, followed by several coats of good marine spar varnish with UV protection.
If possible, some cleats in the back mounted in a sliding dovetail joint can help. The cleats need to float, to prevent the sign from splitting due to seasonal moisture content changes.