Hi, we stayed at a hotel at Campo Santa Maria Formosa during our second time to Venice (have been to Venice multiple times). It’s a nice spot away from the Rialto-to-San Marco route. It’s a bit tricky to find from the vaporetto stops, so look it up on a map (maybe even print it), so you aren’t backtracking over bridges with your suitcases. I would take either the Aliguna boat (the orange line - arancio) from the airport or the vaporetto down the Grand Canal if you’re arriving by train. Go to the Rialto stop and walk from there.
While you’re printing a map, add Suso’s to it if you like gelato. It’s between you and the Rialto Bridge.
I couldn’t figure out why you’re taking the vaporetto over to San Zaccaria?
When we stayed at Campo SMF, we rarely took the vaporetto since it’s easy to head from there where you want to go without taking a vaporetto around the outer perimeter. Definitely don’t take one from the Ospedale near you around the Arsenale that finally reaches San Marco - not a pretty route.
The Palazzo Contarini del Bovolo is in your vicinity, too, and now you can buy a ticket there to walk up the outer spiral staircase.
Be sure to take a vaporetto in the evening after the day tourists have left. You could ride down the Grand Canal from the Rialto and getting off at San Marco.
To head back to the train, walk back to the Rialto stop and catch the vaporetto to the train station.
I know you are concerned about crowds. I was in Venice again last June and August. Yes, San Marco piazza is a swarm during the day, but unless you’re at that spot or along the Rialto/San Marco route, it’s pleasant, especially crossing the Accademia Bridge and exploring the Dorsoduro neighborhood. Or just stop for a bit, sit at an outdoor cafe and watch the scenery. I reserved a ticket on-line ahead of time to go into St. Mark’s - selected the first opening time. The church was quiet this time, and I was able to completely enjoy the gorgeous mosaics. The museum above it is worth seeing. Also, we’ve enjoyed the Clock Tower tour which lets you see the back-side of the mechanisms, a great view outside at the top, etc. The English version tour is at 10am (I think). Reserve on-line ahead of time.
Have a great time! It’s a unique & magical city!