Trenitalia tickets

Yes, you are correct, assuming you are landing at Rome Fiumicino:
1) Local train from Fiumicino to Roma Termini. ~30 minutes
2) High Speed train from Roma Termina to Venezia Saint Lucia. ~4 hours

You can look up times and see prices on the Trenitalia site here: https://www.trenitalia.com/en.html
You can usually buy tickets up to 120 days in advance, it does vary. However in this case as you cannot predict when you will emerge from arrivals, best not to book in advance, just buy tickets when you arrive and pay full fare (€100-€106 for both trains).

"Found it somewhat confusing to understand from the website which trian stations were best as there are more than 1 option per city." - the website will always show the quickest option. Just look up "Fiumicino Aeroporto" to "Venezia S. Lucia"
The other station in Venice (Venezia Mestre) is on the mainland, not what you want.

OMG Don’t ruin your first Venice trip by staying in the mainland in Mestre. You want Venezia S Lucia station on Venice island.

If you want to avoid missed connections then your best bet is to stay your arrival day in Rome. Planes can be late due to weather, mechanical issues, etc. You'll need to give yourself a margin of time to account for passport control, taking the train into Termini, etc. Train tickets purchased in advance have a good discount, but if you miss the train not only are you out the cost of those tickets you'll need to buy full fare ones to get to your hotel in Venice. By spending the night in Rome you just go into town, drop your bags at the hotel, then start exploring. And you're sure to make your discounted train the next morning.

If you want to avoid missed connections then your best bet is to stay
your arrival day in Rome

This is good advice if you have a nice long stay planned. However, for those with limited time, getting to Venice on your arrival day saves one hotel change and valuable non jet lagged time. But the trade off is cost- buying a full priced ticket on arrival instead of a discounted ticket in advance. Only the OP can decide which is better for them.

If you haven’t purchased your airline tickets yet, try to fly open jaw, into Venice and out of Rome. Just choose the multi-city option when purchasing your tickets.

Philip's got it, and it probably won't be any more expensive than a simple round trip once you reckon in the costs of the travel by train. Easy bus connection from Venice airport to the city..