The adapter isn’t expensive and it’ll be more than strong enough for 2 bikes.
I personally wouldn’t trust the rear suspension of a Prius for a 4 or 6 bike crazy mount. Decent chance that it is strong enough but you’ll probably surpass the tongue weight limit of a Prius.
Amazon sells the adapter for $35. They have an extra bracket to keep it from wobbling for $12, that probably functions to seriously slow down rack thieves (I was told they exist).
The Prius AWD is likely to be my next car. I recently moved though and found myself driving the second (beater) car about 20 miles a week, so I’m in less of a hurry than I used to be.