Can I ride my S-pedelec without a display?

No, for legal reasons, S-pedelecs must be equipped with a speedometer that is always in the rider's immediate field of vision. Kiox 300/Kiox 500 meets this requirement: the display is attached to the handlebars while riding, so the rider always has the current speed in view.
S-Pedelecs must not be used with the Intuvia 100 or SmartphoneGrip. Both are connected to the eBike via Bluetooth, i.e. wirelessly, and can therefore also be carried in a bag or rucksack. For S-pedelecs, the requirement of a speedometer that is always in the field of vision is therefore not fulfilled.