Venice restaurants in San Marco area - non touristy

Around San Marco, not so much, since it is tourist central. You will need to go into more residential areas to find local's eateries.

what Sam said

Agreeing with the previous comment. Most locals don’t live in the San Marco area. I don’t attempt to have great meals in Venice & save those experiences for the other Italian cities.

But I absolutely love Venice! : )

Just over the bridge in Castello, is a small piazza, Compo SS Filippo e Giacomo, with local shops. My B&B hosts took me to this restaurant.
I fell in love with pane e baccala mantecato. It sounds awful but is so good, traditional Venice food.

EDIT: Restaurant is Aciugheta. Website is https://www.aciugheta.com/en/
Sorry it didn’t link the first attempt.

The “Page Not Found” restaurant?

I agree with Jean - don't attempt to have great meals in Venice. Having said that, Rick's general advice to walk a ways off the main drag holds true here too. While you wander around and get lost, keep your eyes peeled for smaller restaurants without English menus in the window. You may well end up eating away from San Marco in a different neighborhood, but the island is small so that's ok. Your hotel or rental home host may have a tip or two as well.