Revving but no Acceleration

Just recently began encountering this problem. I drive a 2004 Prius with 160k miles, the code reader I have is either too cheap to read my codes or the car won’t give them, another thing I might be curious about.

After driving for what has become shorter and shorter distances the car changes in behavior. The hybrid battery seems to work to give power as it is meant to, but the engine will stop providing acceleration, leaving the car struggling to get over 20mph. I’d describe it as slipping or not being able to get the car to “shift” above first gear, however I am already aware that the Prius does not have a clutch. The engine revs to very high rpm when the gas pedal is pressed but again, no power being contributed to moving the car, leaving it weak and unable to get me anywhere. Another thing I notice is that when this condition presents itself the car doesn’t decelerate from the minimal speed it is able to achieve as well as it typically does. It seems to want to maintain these low speeds, and I even coasted a good flat 200 yards with no deceleration to the normal idle speed. When I turn the car off and on again the problem goes away, but as stated this solution has become more and more temporary.
So far I‘ve only changed my transmission fluid, which seemed to be overdue for a change.
Any ideas what might be wrong and what I might need to do maintenance on or prepare myself to have repaired or replaced?