Vienna > St. Gilgen (via Wachau Valley) > Lienz (via HAR) > Innsbruck

Planning depends on how thoroughly you want to visit Wachau valley. If you want to see all what you mentioned, including a "couple" of wineries, you should stay for a night somewhere. You must do the math; most people underestimate the time needed.

Some rough estimates:
Driving from Vienna to St. Gilgen is about 3 hours (without stops).
Visiting Krems, two hours and more, walking in the old city, visiting museums on the Museumsmeile (museum's mile).
Dürnstein, two hours and more, climbing to the top of the fortress' ruins, medieval town walk, maybe a posh lunch or dinner at Richard Löwenherz restaurant.
Melk Abbey, three hours and more, depending how much of the premises you can take during a single visit.
And so on.
(It is difficult to visit a winery, do some wine tasting, and to consistently spit out the wine because you are driving.)

In Langenlois, a town about 8 miles NE of Krems, you can visit the Loisium wine world:
https://www.loisium.com/en/world-of-wine-langenlois/home/

On your day on the Grossglockner, after reaching the Pasterze glacier, there is a tunnel that will bring you to some nice views of the glacier. I understand that there is a hut at the but we didn't have enough time on our visit to make it to the end.

Also, look into Krimml Falls on your way to Innsbruck. There are some taxis(across the road from the ticket office) that will take you to a point at the top of the falls if you would rather walk down than up. The fee to view the falls is covered in the taxi fare.

Overnight at Maria Taferl is a great idea, but visit the Pilgrimage Church before going to Melk. The Baroque churches at Durnstein, Maria Taferl, and Melk were all designed by Jakob Prandtauer. Maria Taferl will blow you away IF you see it before Melk.

don't rush the drive to St Gilgan. Linz isn't worth it, but in the vicinity see at least Mauthausen Concentration Camp and the Wilhering Abby church. And a day side trip to Chesky Krumlov will not be regretted.

The Gosausee is a must. Can be done in a day with Hallstatt.

Salzburg: First time we went, easily got free parking by the Mirabel Garden. Second time, street parking for $8/hour on the fringe of downtown. Third time, used the underground lot, which is the only thing you should consider. Bring lots of $$$$. The only good way to get to this parking is from the expressway west of the city

Go to the Grossglockner via Salzburg and Lofer. Lofer is our favorite quaint Austrian village although a bunch of new condos at the ski base have degraded the ambiance. Explore the pedestrian only small old town. Also, Lofer has been our top choice for souvenir shopping because there are a couple shops on the square by the church that seem to be outlet stores for Austrian fashions, especially Loden coats. Loden is breathable highly water resistant wool. To make Loden, a bolt of wool fabric is boiled and then set out in the sun to dry, which causes it to shrink which squeezes the threads to tighten up making it difficult for water to penetrate the breathable fabric-- think of Goretex as synthetic Loden. Geiger is a well known Austrian fashion house. Geiger makes loden jackets. My wife got one in Lofer for $65. A couple weeks later, we saw it in America for $385.

I don't know if Pustertaler High Alpine Road is worth a day. You don't say when you are going, but I'm given to understand that the Grossglockner Road is very crowded in high season and it is close to impossible to get a parking place at the glacier view. Alternatively, if I remember correctly, there is a year round ski resort near Zell am See where they ski on a glacier. Unlike Grossglockner, where you look at the glacier, at this place, you can take the ski lilt up and walk on a glacier (also on the Dachstein near Hallstatt). .

Get a Michelin Green Guide (the best road trip guides) to Austria and 1:200,000 road map-- Michelin, Kummerly & Fry or Bartholomew

What great feedback! Thanks.

We are travelling from 26-Aug > 5-Sep

Definitely staying the night at Maria Taferl. Looks like we will have plenty to do over there in the area.

@kb1942 i think you got confused - i wasn't referring to Linz (enroute to St Gilgen) but LIENZ which is down south after the drive through HAR. I was asking if it makes sense to sty 2 nights in Lienz. But, after a bit more research, this is what i think might be feasible:

St Gilgen to Grossglockner HAR, taking our time and eventually arriving in Lienz late afternoon. Hang around town. o/n Lienz
Next day, drive around the area. There is a Osttirodler etc. In the afternoon, we do the loop around The Pustertaler High Alpine Road and o/n in Lienz. (so that's 2 nights).
Next morning, we head out to the Hohe Tauern National Park, spend the first half there, have lunch at Mittersill, head over to the Krimml Falls and then finally make our way to Seefeld for the night. Is that too hectic?

Linz is in your way. My point was don't bother stopping, but also consider that there is some great stuff to see in the vicinity of Linz.

The 3 mountain valley roads just west of Innsbruck-- Oetztal, Pitztal, & Kaunertal are 3 of the best mountain drives in the Alps. I can't remember if it took me 2 or 3 days to do them. If you can't fit these in, skip Pustertaler High Alpine Road & the second night in Lienz

Once you get off the expressway into the mountains, figure on traveling 30-60 km/hr. With the recommended stops, Michelin, estimates a full day for the 75 km Grossglockner road. You probably are biting off too much.

Dear fellow travellers,

We are back from our trip and i haven't stopped smiling. I thought it might be useful for others to get some idea of what we did and our +/- from the trip.

Vienna (3 nights)
Stayed at Hotel Imperial. Stunning! Great Location. Everything was walkable.
Loved the Food. cafe Central was great
Demel was average.
Cant remember the name of the place but a small cafe that server Pfiff (1/5 beer) and open sandwiches. Delish!
Schoenbrunn Palace was pretty amazing.
Would love to go back to Vienna again.

Drove through Wachau Valley. Stopped along at Krems an Danube, Durnstein eventually at Maria Taferl. What a delightful little town. 1 street. 2 hotels. We stayed at Hotel Rose (Overnight). Breathtaking views of the Danube. Church was stunning,
Melk Abbey was great.

Drove to Sankt Gilgen (4 nights) as our base. Just drove around to Salzburg, Post Alm, Bad Ischl, Altausee and most evenings in St Gilgen. This was our fave part of the entire trip. Avoided Hallstatt - so so so touristy. lost its charm id say.

Drove down to Grossglockner HAR. On the way, stopped at Fusch and was lucky enough to catch the seasons first Almatrieb parade. The HAR is such a magical drive. lots of stopping along and enjoying the views. Did go up to the glacier. It is insane, but also sad to see the effects of global warming and how quick its receding.

Got down to Lienz, spent 2 nights here. Was actually very nice here.

Next day (planned) was Hoehn Tauern & Krimml. But the rain gods had other plans. So we canned that idea and drove straight to Seefeld (noth of Innsbruck) and stopped at a couple of towns along the way (Alpbach).

Innsbruck by itself was our least favourite. Just a busy city with lots of people and touristy stuff.
Achensee, Mittenwald, Achenkirch was absolute pleasures to visit.

Drove a Manual (Diesel) car all the way through and loved it.

Thanks for all your help during our planning.

can't wait to get back to Austria again ! So much more to do.

What do you mean by HAR? Is that a German or English acronym?

High Alpine Road (Großglockner Hochalpenstraße)