Prius overheating on highway. Changed thermostat. Water pump is operational.

Yeah, I'd test it in boiling water first to see if it actually needs replacing. Its like $40 at autozone bc it comes with the housing. Also, maybe you could test your coolant flow system like I did mine:
Disconnect lower radiator hose entering thermostat. Then wrap a bunch of tape around end of garden water hose to create seal grommet plug then ram the hose into the big thermostat inlet and force hose water through thermostat after a bit of time to allow some engine cooling. It will get difficult once the temp goes down and thermostat closes, but if you force the homemade plug and hose end into inlet, water pressure will overcome spring allowing water to flow freely through motor and hopefully all the way through the radiator and back out the lower hose you disconnected. If water is coming out of the removed rad. hose, your flow is good. The liquid should get warm within a few seconds and both hoses along with the radiator should feel warm. If you can't get warm water out of the lower radiator hose, you've maybe got a clogged radiator or something else. But Im no expert. All that just seems basic water flow principles. Hope that helps. Also, You could disconnect the other radiator hose and push the water through the lower hose possibly forcing out sediment that could be clogging radiator. But make sure you disconnect the other side of radiator so you dont push the sediment into your engine.