Itinerary help - Venice to Dolomites to Austria and back to Venice

As a general rule, weather will dictate your actual plan, no matter how much you plan in advance. You should have already discovered it two years ago. July could be better than May but there is no guarantee.

As a general rule, roads in the area are superb but not fast. In days when you are driving, do not plan too much: you just need to end driving on the trail of a slow truck and you will have to add one hour to your travel time. The day you are driving from val Gardena to Cortina area will be a long drive but an extremely scenic one, plan some extra stops but not extra hikes.

The drive from Hallstatt to Venice involves a long road tunnel that sometimes has regulated access and queues may develop. Add prudentially one hour to your plans. Even the Venice metropolitan area has often intense traffic. You should better stay on the mainland as getting to the airport would be much easier, but this conflicts with your idea of visiting Venice in the late afternoon. It looks to me you are asking too much to a single day.

I am afraid you would need a printed map to have an idea of what you are looking for. Grossglockner and Hallstatt are in central Austria, Salzburg is almost in Germany, Innsbruck is in western Austria. You must strike a balance or you will end driving all the time.

Remember that fuel is much cheaper in Austria than in Italy, and that you need a motorway tax vignette for Austria.

This fall we stayed in the Dolomites and drove through Austria after (looked into staying in Hallstatt) but choose Lake Bled instead and then went to Venice and stayed there. Main reason we did so was the driving time from Hallstatt to Venice compared to Lake Bled to Venice.
To get from Val Gardena to Lake Bled you have to drive through Austria. Most of the entire drive is in fact in Austria between these places.
I think you may have an overly romantic impression of the main driving roads.

I thought the scenery in the Dolomites in the mountain pass areas was far more impressive than in the parts of Austria you will be driving through but for the most part the Austrian drive will look identical to the Dolomite drive when not on a mountain pass type road . It is not like you cross the border and it looks different, the drive is scenic but not overly impressive.
Expect highway sections to look like highways.
Mountain pass driving is stunning in comparison but not the fast way to get to point a to point b so I don't consider them main driving roads and you will have left the mountain passes behind as soon as you leave the Dolomites headed to Austria.
Any mountain passes in Austria other than the Glossglockner which you are already planning would be way out of your way to include.
You won't stop for pictures that often.

My thoughts:
1.) If flying from the US to Venice ; driving all the way to Val Gardena that same day is not that great a plan since you will be tired.
2.) I would stop trying to find more destinations in Austria, you already have your top 2 ; see those and be done. Nothing else nice would not involve going out of your way and I don't see you having the time or the need to do so.
3.) Glossglockner is 3-4 hours drive away from the Dolomites, so if you want to do that you might want to spend a night near there since it is another 3+ hours from there to Hallstatt.
From what I understand the Glossglockner is never on the way, it may be a not that far off route side trip.
We drove through and stopped in Lienz which is a close town and I think the largest town near the Glossglocker ; still a very small town. Small towns in Austria are really tiny not sure I would even call many of them towns. I think in night in Lienz probably makes sense for you but would do the Glossglockner ride first and would view it as a place to lay my head only and get to Hallstatt as soon as I can.
I was looking for a little gem of a Austrian town along a similar route and concluded one did not exist, Lienz was the closest thing and many Dolomite villages I would have preferred.
4.) If Hallstatt is so high on the list spend at least 2 nights there, not just 1. If it were me I might power through a really long drive and stay here 3 nights, skipping staying the night in Lienz. As landscape photographers you know you cannot depend on good or even decent light if you are going to place and have 1 chance to photograph it ; one night stays should be avoided in any area of importance to you.
5.) Venice you are leaving no time to see at all. If you get there late afternoon and have to leave the next morning I would not even bother to stay on the island of Venice. Too much expense and added hassle.
I would rather spend at least 2 nights and stay on the island
Assuming 9 nights is what you have, I would revise to:
4 nights Dolomites - 1 in a rifugio / 3 in Ortisei ; since Cinque Torri is closer to Venice than the Val Gardena better to stay here the first night.
1 night in Lienz or skip and drive straight though
2 nights in Hallstatt or 3 in Hallstatt if you can make the long drive all in 1 day
2 nights in Venice

Looking on a map, after Glossknocker if Hallstatt is too far, another scenic lake town like Zell am See might work for an overnight and then head to Hallstatt the following day?

For the drive you are planning:
1.) make sure to tell the rental company when making the booking your plan to drive to Austria - don't just assume it is ok
2.) once you cross the Austria border, get a highway vignette from the nearest gas station. Easy to do and not that expensive, not doing so you can be fined large amounts with the ticket payable on the spot no exceptions so potential nightmare scenario if you don't have much cash on you.

There aren't many areas that offer better scenery than Austria.