> There are clearly good reasons for people to want extremely powerful e-bikes and they should be allowed to.

They are called motorcycles. At > 4kw they are that (here). So either you get them registered a such, get a license and insure them, or downgrade them to under 4kw, get a license and insure them as a moped, or downgrade them to 2kw and pedal assist only and register them as a pedelec. All other options is 250w continuous (you can get away with about 500w peak) and pedal assist only.

You are also not insured if you drive an illegal bike on the road.