Use a neutral density (ND) filter. It reduces the amount of light entering the lens without changing color, which lets you keep a wide aperture and slower shutter speed in bright conditions.

For your case, an ND filter is the standard solution when ISO is already at minimum and you still have too much light. It’s especially useful for panning or showing wheel/background motion in daylight while keeping depth of field shallow.

Other things that may help, depending on your camera:

• Use your lowest native ISO.
• If available, use a faster shutter only when needed, but that works against the motion effect you want.
• Stopping down the aperture would reduce light, but also increases depth of field, so it doesn’t match your goal.

A circular polarizer can reduce light a bit, but it also affects reflections and is not as predictable for exposure control as an ND filter.

So the main answer is: fit an ND filter with the strength needed for the light level, then choose the shutter speed and aperture that give you the motion blur and background separation you want.