Venice advanced tickets question

Always avoid third party sites for anything: tickets, tours, airlines, and/or hotels.
There are many stories of third party cancelations at the last minute. These vendors often sell tickets that they don't hold yet, and hope to secure. Good luck!

Book a timed entry to St Mark’s on the official site to avoid the ticket line
Nobody skips security lines
Take note of entry restrictions-large bags not allowed
Shoulders and knees must be covered-both women and men
They are very strict

Other than that-maybe the Guggenheim needs advanced purchase-depends on what time of year

Avoid third party resellers

I generally agree with the answers above. However, there are times when tickets or passes on platforms like GetYourGuide cost exactly the same as on the official websites .. with the advantage that the booking process on GYG is often much smoother and more user-friendly than on the official platforms. In some cases, you also get free cancellation, which official sites may not offer. Plus, certain experiences are only available through booking platforms. Yes, they get a cut, but the offer a service in return. So it's not always balck-and-white, that's my point.

Beyond the attractions already recommended for advance booking, I think that should be enough. And even if a venue ends up sold out or too crowded, don't worry because Venice has so many other things to discover, practically around every corner.

Whether to do a tour or not really comes down to personal preference. If you enjoy DIY travel and have a solid guidebook or app and like to do your own research, you can definitely do Venice independently. Tours add context and storytelling, but you trade some freedom for that structure. Personally, I rarely go on tours, but a few of the ones I've done were absolutely worth it and cannot be self-made: like the after-hours tour of St. Mark’s or the terrace at Palazzo Pisani.

Sites in Venice where advance booking is useful/critical - Other than Doge's Palace, I would say just Basilica San Marco, if it's busy season.

Some attractions are worth getting a guided tour, but a lot of other stuff (especially the places with RS audio tours) can be done on your own just fine. Audioguides are usually available but often extremely basic. I like to do a mix of DIY and guided tours, splurging on good guides for the really large sites with a lot of hidden history (e.g.: Vatican museums, Borghese gallery).

I personally try to buy tickets directly from the official seller if possible, because there are a lot of sites with official-sounding names which mark up the price. It can be really difficult to differentiate the original from an impostor. Even with an impostor site, you'll probably get a legit ticket (it's not a scam exactly), you just may wind up paying significantly more for no reason.

GetYourGuide/Tripadvisor/Viator etc. are just 3rd party resellers, they sometimes mark up the price but not always. If you want to use them to buy a tour that you could not get otherwise on the official website (for example, a day or half-day tour with minivan to see multiple sites, city walk tours, etc.), then it makes sense, but additional layers of administration usually makes things worse if anything goes wrong. I've heard from people who were unable to get refunds from the reseller when sites were closed on short notice. The one real benefit from using a reseller is that they usually allow you to cancel for a refund until the day before the event. The official sites almost never allow refunds or exchanges.