A better solution might be capping the power output of the rider and motor combined at something like 300W (ie a good sustained effort for a strong cyclist), and disabling power assist if the rider breaks that threshold.

This is almost exactly what is done in the country I live in and it seems to work, the only difference being that it's capped at 250w rather than 300w

So that's what I was trying to say, but I should have been clearer lol