I was not familiar with that campo and had to look it up on Google maps. It is indeed very close to San Marco but not on a tourist path. During the day you could easily move around the city by walking straight north, away from San Marco on the back streets. In the evening, after the ships have left, you could stroll down to the water and enjoy the views. The area right in front of San Marco, and the square itself, still attract people ( and trinket sellers) Innthe evening but you can still find some open space.
Does the other apartment look right AT the Rialto market? Not sure I would like that. It is very crowded during the day--hard to walk through--- and then the fish area is deserted (and wet) at night. I believe Rialto Bridge itself is under renovation and covered with netting and scaffolding, and the walking path may be narrowed. We will be there in October ourselves, in an apartment a bit farther from Rialto Bridge but on the market side. i learned of the renovations after I booked the apartment.
It seems the A/C is a main concern? I don't know that it should be. We spent the entire month of October in Venice two years ago and kept the windows open the whole time---nice breezes and no mosquitoes. Indeed I don't remember if the apartment even had A/C, and if it did, it may have been past the allowable time to use it ( this is restricted n Venice to conserve energy).