...are there 300k mile Priuses out on the road and being used?

The indications from several articles is Yes.

eg, from one:

  We clarified that the standard mileage for the Toyota Prius Prime is up to 500,000 miles, but we would place the high mileage point for the car at around 300,000 miles. Once the vehicle passes this point in its lifespan, it’s far more likely to experience issues that cost ample money to keep in excellent condition.

I've been in several Prius taxis with more than 300K on the odometer.

Priuses are used for taxis all over the place and 300k miles is pretty standard.

Oh yeah. There is a famous one in austrian taxi service with over 600k miles.

https://www.electricbike.com/the-curious-case-of-the-600000-...