We're taking a cruise ship out of Trieste in October. We will fly into Venice and stay 3 nights.
Then we will need to get to Trieste from San Marco with our luggage. We are two very senior ladies.
Bus would be your best bet. It’s about 2.5 hours. Flixbus has a route, flixbus.com
Train also works. There are direct trains from Venezia Santa Lucia station.
Train would be more comfortable, and they run quite frequently, so you are not bound to the sparse and unreliable Flixbus schedule. Just don't take more luggage than you can handle on your own.