Driving from Dolomites to Zermatt

Google maps provides two alternate routes, but they add up to 2 hours to the trip.

Where are you renting the car? The route via Innsbruck, Feldkirch, and Andermatt will require vignettes for both Austria and Switzerland if the car was rented in Italy.

We are renting the car out if Frankfurt airport or vicinity.
I checked Google maps also but am uncertain about driving through Milan vs more countryside through Switzerland

Unless you luck out and get a car with a Swiss vignette already attached, you will need to buy one as soon as you enter Switzerland. They are good for 1 year. If you go through Austria and use a motorway, you will need one there as well. Austria has short term vignettes for less money.

https://www.ch.ch/en/travel-and-emigrate/holidays-in-switzerland/motorway-vignette/

https://www.austria.info/en/service-and-facts/getting-there-around/by-car/vignette

Since you are renting from Frankfurt, you will need to get a vignette for that little stretch of Austria anyway. The shortest duration is 10 days. Expect delays going through the Brenner Pass, where you have to pay a separate toll. You can pre-pay the toll as soon as you know your rental car license plate. Then you just use the special pre-paid lane. A camera recognizes the license # and opens the gate. https://vignetteaustria.com/brenner-motorway-toll/

If you go back north via Innsbruck to Tasch, you’ll go through the Brenner Pass again.

We drove from Munich to Ortesei, Ortesei to Venice, Venice to Stresa, then Stresa to Lauterbrunnen in June. So a good portion of the route from Ortesei to Tasch via Milan. It’s easy driving, with a few toll booths in Italy. We were in traffic at the Brenner Pass for about 30-40 minutes. Driving through Italy was mostly countryside. The highway bypasses cities. There are not a lot of food options along the highways. You need to exit and go into the cities, or at least suburbs, or stop at an AutoGrill - truck stops which serve surprisingly good food.

We took the auto train at Iselle, because a couple of us get carsick so we were avoiding too many mountain passes. But you can drive over the pass instead.

Whatever driving distance the map says, I would add an hour or more. Even more if you want to stop at any of the gorgeous places along the way.

Going via Milan is, by far, the most "comfortable" and easiest route. It is also rather uninteresting (except Simplon pass).
However, the only real scenic alternative that I see involve a long day of driving (9+ hours) over several mountain passes, where having two non-nervous drivers would be required. It basically travels west to Zernez (CH) then over the Albula Pass, the Oberalp Pass and the Furka Pass. It is magnificent, but it is exhausting. If you drive north through Austria you skip the first third of mountain driving, but it is significantly more expensive and the Arlberg Tunnel is currently closed anyway.