There are plenty of ebikes with throttles that have less power than ebikes without throttles. E.g. a lectric xpress 500 has 500 watts, there are some pedal assist only bikes with 750 or more watts.

It's power and speed that matters, as you point out, so make regulation built on that. Heck, arguably pedal assist is a throttle, it's just a different mechanism vs a twist handle.