Edit: Just follow Sam's advice above.
regional trains are safe to book once in Italy,
There are no reserved seats on Regionale trains, so you can't actually book them. You can only pay Trenitalia in advance. Then you'll have the same chances to get a seat of the guy who bought the ticket via app 15 minutes before the departure.
and “fast” trains should be booked ahead of time.
Should be booked in advance To save money. 2nd class seats on Fast trains rarely sell out, if it happens you travel in 1st or wait for the next train.
There is one relevant exception to this rule: Trenitalia's IC trains going to/from the seaside on summer weekends. Unfortunately, July 30 is a Saturday.
any suggestions on routes
I'd try to make less transfers and have reserved seats all the way to Venice.
You can Take the IC train from Monterosso to Milano Centrale and then an high speed train to Venice. www.trenitalia.com is programmed to display only the shorter routes and this one is a little longer, You'll have to make two separate searches and two purchases.
Pick a transfer time at Milano Centrale longer than 15 minutes, this way if you miss the connection because of Trenitalia you will always be protected. This is true even if you purchased a discounted Trenitalia ticket for the fast train to Venice.
If you miss a connection when transferring into a fast train, you must walk to the ticket booth by the tracks and get a new seat reservation for free. Not rocket science.