My 2019 Prius Prime has about 90,000 mile now. I put new tires on at 80,000 because it felt like the tires were out of round or out of balance. Still had the same vibration problem with the new tires although not as bad. Changed transmission fluid with Toyota WS and the vibration went away. Much quieter and smoother now. I wouldn't think that transmission fluid would have any effect on vibration. Any thoughts?
Was the steering wheel shaking at over 60mph? Usually that that would mean a wheel is out of balanced. I noticed you said you put in new tires at 80k miles in just over 1.5 years for a 2019 model year, I'd say the tires was the issue.