Question about Continental Grand Prix 5000

I ride a 30 mm Grandprix 5000 with an inner rim width of 18 mm

In my opinion, there are no manufacturer specifications for this. The shape of the tyre changes slightly depending on the inner width of the rim. The narrower the rim, the more bulbous the tyre. If the rim is too narrow, a wide tyre may become somewhat unstable when cornering.