Thanks for your message and sorry for the delay.
Orvieto is beautiful and convenient to visit on the way from Pienza to Rome, you can leave Pienza and head towards Montepulciano to take the A1 Highway to Orvieto (about 1 hour and 15 minutes driving).
This post of mine about a road trip from Rome to Tuscany and bacj may be of of your interest,
feel free to contact me again,
Have a nice stay,