Help with train reservations in Italy, August

A more efficient route would be to travel Venice > Florence > Riomaggiore > Orvieto > Rome. The trip from Venice to the C.T. will take about six hours, so basically a full travel day.

You should be able to buy tickets now for any segments using fast trains such as Freccia or InterCity, which will save some money if any of the cheaper tickets are left. Note that tickets for the fast trains are specific to one train, date and departure time, and the cheapest ones are non-refundable and non-changeable.

You won't be able to buy Regionale segments now, and even where it is possible to buy these in advance, they come with restrictions. It's just as easy to buy Regionale tickets when you arrive in Italy, either at a staffed ticket window or from a Kiosk.

Globulen,

You may want to refer to this recent post to read about your options for travel Venice > Riomaggiore:
https://community.ricksteves.com/travel-forum/italy/train-from-venice-to-cinque-terre

You may want to change your route as it makes more sense in terms of travel; Venice > Florence > Riomaggiore > Orvieto > Rome.

You can buy your high speed train tickets in advance (up to 120 days) at a discounted price but keep in mind that these come with restrictions (unchangeable & nonrefundable)

You don't need to buy your Regionale tickets in advance, as there is no price advantage, & they come with restrictions.
You can check train schedules on the trenitalia website.

Enjoy your trip!

Edit: Ken posted while I was writing this post...

Trenitalia still has not loaded the Regionale schedules for dates beyond June 13, the semi annual European train schedule adjustment. So if you want an idea of what to expect, just look at the same day of the week you expect to travel within the next 7 days. That will give you the price and times. They usually don't change too much. Keep checking, they should be out soon.

Thanks for all the great responses! Very helpful!