Thanks for your answers, much appreciated. I will also try to inflate without the valve cores, see what happens. If the recommended tire pressure cannot be safely achieved with these rims and a larger tire size is recommended, it is a mistery for me why Giant then mounts 25 mm tires on the bike (or did I misunderstand something in your comments?).
It will take a bit until the electronic pressure checker arrives, but I will report the results. Maybe some other readers have the same issue..