The information I'm providing here is from our February 2025 visit to Italy.
You don't need to buy tickets in advance to Orvieto.
The route I know of would be to take the Leonardo Express to Termini train station. Buy tickets there. A few things to note, the RS guidebook makes it sound like its a quick trip from Rome to Orvieto. Not necessarily, was the case for us. Depending on time of day, the trains may not be hourly. Some trains connect in Orte and from there you take a bus to Orvieto. From Orvieto train station, you cross the street and take a funicular up to the city, and then either walk about a mile? or take a bus to the historic town center. All those transfers are easy, but transfers none the less.
With luggage, you might look for the routes that go to Orvieto directly from Termini. Be aware, that at least some of the trains to Orte and Orvieto, leave from 2est, which is a longish walk from the main part of the station.
We LOVED Orvieto, so don't let the logistics discourage you.
I wouldn't buy a train pass. I would buy regional trains day of with the app or at the station. You might look at the pricing of the high speed trains and determine if it makes sense for you to buy those tickets in advance. In general, tickets purchased in advance are not refundable/changeable.