Hello Cissy K,
The St Marks Basilica tickets can be purchased here: https://basilicasanmarco.skiperformance.com/en/store#/en/buy
In my opinion you should buy the St. Mark's Basilica plus Museum and the Loggia of the Horses tickets. The extra purchase gives you access to more upper levels of the cathedral for different views, access to the interesting museum exhibits as well as access to roof to look out over the piazza and the Grand Canal.
The Pala d'Oro is probably only really interesting if you're altars and church decoration. You can buy the upgrade to these areas while inside the cathedral itself so you don't have to decide before hand but that's my vote.
St Mark's tickets are separate from all other offers and sites so you'll need to buy them on their own. There are many different Venice discount and museum cards but I don't think you'll be there long enough to take advantage of any of these so i would just buy tickets.
The Dodge's Palace ticket information is here: https://palazzoducale.visitmuve.it/en/pianifica-la-tua-visita/tickets/
In my opinion the Dodge's Palace is a visit that really gains from context and historical information so a tour of some kind may really help you to appreciate the history and machinations of old Venice. There is the Secret Itineraries tour that supplies some of this - a close look at the cells, administrative centers and torture chambers with Casanova's cell. I don't know what the options are for other tours but a good tour guide could really enrich the experience. The palace is amazing on its own and worth seeing but it means more if you understand the treacherous society it represented in old Venice.
You can do both in a long day - Dodge's Palace early and St Mark's in the afternoon, but it will be a long day. If you do this think about Rosso Pomodoro for lunch. It is a Milan fast food chain with a huge (for Venice) dining room near the square and American speed service which is important if you are looking to make an afternoon ticket time. Otherwise I would recommend getting take away.
https://ristoranti.rossopomodoro.it/venezia-b7927c480a78
I love Venice, have a great trip,
=Tod