Can I also ride my speed eBike without a display?

No, speed eBikes must be equipped for legal reasons with a speedometer that is always in the rider's direct field of vision. Kiox 300/Kiox 500 meets this requirement: The display is located on the handlebar while riding, so the rider always has the current speed in view.
Speed eBikes must not be used with Intuvia 100 or the SmartphoneGrip. Both are connected to the eBike via Bluetooth, i.e. wirelessly, and can therefore also be carried in a bag or backpack. For speed eBikes, the requirement of a speedometer that is always in the field of vision is therefore not met.