Saila: What was disappointing about Yosemite? Some of the most amazing scenery anywhere IMHO, but Zion is right up there as well.
DocT: Really look at each park’s website for information about how long it will take you to drive from one spot to another. During the warmer weather, many parks do road maintenance which can add a half hour to any trip. Even without road closings, we were surprised at how long it took to drive around Yellowstone for example.
As for how long to stay, it really depends on what you want to do in each place. Do you just want to see the main sights? Are you hikers? Do you want to move every few days or pick a few places and spend more time knowing you can’t see every site? Do you want to make it all the way to the west coast or consider going south from Glacier through Salt Lake City and Utah to Bryce, Zion, Grand Canyon and northeast to the southern part of Colorado and to Mesa Verde.
Alternatively, skip Glacier and head across from Yellowstone to Crater Lake in Oregon, down to San Francisco, then east to Bryce, Zion etc. Or add in some coastal CA like Big Sur and then head east.
There are many possible options and you really need to decide what you want to do and see ahead of time and how much driving time you want.
When we went to Tetons and Yellowstone, wished we had had more time in the Tetons. But we love to hike. We went through Yellowstone and headed east, stayed in Cody, WY where we saw a rodeo, then went to a dinosaur dig site (had a dinosaur-obsessed kid at the time).
To me Grand Canyon is good for 2 nights, assuming a day and a half or a little more to do some hiking into the canyon. Zion could be several days, or just one or two. Bryce a day maybe to see the main sites.
Mesa Verde in Colorado with the cliff dwellings was great for the southern route. Sedona in Arizona is a beautiful spot, but I think the fall is very busy there so lodging may be challenging if you can’t plan ahead of time. It may also give you some structure so that by the end of the trip you aren’t feeling too rushed.
You may want to plan some stays in the most popular National Parks ahead of time, which you can always change. The Zion Lodge, for example, is almost completely sold out for October (but as I recall there are options in the town at the entrance to the park).
Any thought to renting a small RV for the trip? On my retirement wish list!