Your trip is coming up in December, correct?
You haven’t asked about advance reservations, but I will suggest a couple.
The Basilica San Marco has a long line, so “skip the line” reservations are recommended, even in the off-season. I just checked for availability and there are no reservations available on the official website until October 31. Tickets for early November are about half sold. December still has lots of availability, ut many tickets have been sold already.
Book on the official website here:
https://basilicasanmarco.skiperformance.com/en/store#/en/buy?skugroup_id=2524
You have to select a ticket type from the options listed, and I suggest you choose Basilica +museum, so you can go upstairs to see the museum and especially the famous horses. You also have to choose a time. Around 11:30 to noon is popular, as that is when the interior of the church is lit and the mosaics really shine. The early slots are also good, as the interior is much less crowded.
For the Fondaco dei Tedeschi rooftop terrace, reservations are mandatory. They are released 21 days in advance, and when I was looking for tickets for late September I saw they were all booked with 15-20 minutes of the time they were released. Now it is not quite as crazy; I see that tickets have been released for dates up through 12 November, and there are still some tickets available for Nov. 6-9, early in the day. But Nov. 10, 11, and 12, the weekend dates, are all sold out.
Going late in the day for sunset is my preference, and those tickets sell out first. Sunset in Venice in mid-December is around 4:30 pm.
https://www.dfs.com/en/venice/service/rooftop-terrace