Don't overthink it. It's a half hour train ride, and the trains run every 15-30 minutes. Don't worry about the trains selling out. You can buy a ticket 5-6 months in advance, and have an assigned seat. That is always a bit of a risk if your plane is late, and has nothing to do with the month of August.

August is not a month when the trains sell out. The 15th is the national holiday Ferragosto, with a lot of people traveling on the 14th and 15th. Those are the only two problem dates. In fact, after August 15th, the trains will be practically empty.

You could take the bus. The grumbling doesn't matter. No buses, anywhere, have seat belts, not even school buses in the USA. People are just projecting from their nervousness about being in a foreign country. Buses in Italy are not known for being dangerous.

Do whatever is most convenient and appealing to you. If you are traveling on the 14th or 15th, then that would make a difference.