Train - when to buy advance tickets

My guiding light for advance train ticket purchases is, I'm OK with it as long as my flights have been booked and my itinerary has been set.

You can look at the train schedules now with a fair amount of confidence that they will be largely unchanged after the June 12 schedule change.
Look at www.trenitalia.com.

You buy advance tickets when you want to save money and your plans are set.
When you need flexibility, you buy tickets before getting on and pay the top, walk-up fares.

I don't know what an "efficient" ride is, but all railway sites display the total journey time and the number of transfers.

I am pretty sure that high speed trains aren’t posted as far out as November. I’ve been watching Florence-Milan and those trains are posted through the first part of September. Regional trains show, but not the high speed.

Right, explore your options on the Trenitalia website. You need to use the Italian names of the city stations you'll be departing/arriving at. For instance, are you leaving Milan after a stay in the city, in which case you'll likely be using Milano Centrale, or are you flying into Malpensa airport which is some distance out of the city, and traveling onward to Venice? Then you'd use "Malpensa Aeroporto" for your search.

Venice's main station is Venezia S. Lucia.
Bologna main station is Bologna Centrale.

Both Modena and Varese have only one station so you can use those names.
You could buy Venice> Bologna in advance if you wish but purchase Bologna> Modena at the Bologna station when ready to depart. That's a trip of under 30 minutes and a cheap ticket.

There are no direct (no changes) trains from Modena> Varese: you'll have a minimum of 2 changes.

Yes, while you can explore the "fast train" schedules that serve cities such as Milano Centrale> Venezia S. Lucia using same days of the week this month as you'll be traveling in November, it's much too early for purchase, even if you could.

I usually just run by the train station the day before we leave and pick up the tickets for the next day in person. By that time, my plans are set and I know what time we'll need to leave. And there are no screwups.

There's nothing worse than getting to the train station too late or having to go through a line--and missing a train. Then waiting for the next available train is wasted time.