If we find it too hectic while there, we are prepared to cut Budapest, have booked a hotel refundable until the day before arrival.
I figure we will have 2.5 days in Vienna & Budapest since the train rides are not too long, & 3 full days in Prague with the 4 night stay.. Short I know

fourfortravel, I can see you are not a fan of Zell Am See, . do others feel the same way?
I kept the 2 nights there so we have the day in between to drive the Grossglockner. If that is not worth doing, we would maybe do one night in Zell Am See & 2 in Hallstat or St Gilgen or St Wolfgang.
But I know an overnight stay in Hallstat is recommended over visiting during the day.

Thoughts anyone?

Only you can decide what's worth doing for you. That said, you are going to a place to do one thing which may or may not be a good day depending on cloud cover which would impair the experience. It's kind of like going to Zermatt for one day hoping to view the Matterhorn only to find out that it's clouded over and you must leave the next day. In the meantime, you're giving very short shrift to the places you mentioned being interested in previously: St. Wolfgang, Bad ischl, St. Gilgan, etc.

My response is in moderation.

Still in moderation? They must be on vacation. Here's my thought:

Only you can decide what’s worth doing for you. That said, you are going to a place to do one thing which may or may note be a good day, depending on cloud cover which would impair the experience. It’s kind of like going to Zermatt for one day hoping to view the Matterhorn only to find out that it’s clouded over and you must leave the next day. In the meantime, you’re giving very short shrift to places you mentioned being interested in previously: St. Wolfgang, Bad Ischl, St. Gilgan, Hallstatt, etc.

Salzburg is touristy. Hallstatt is touristy. Zell am See is touristy and commercial...and expensive. Our children's (wealthy) Austrian friends did not like visiting Zell am See (or Kitzbühel), calling them "chicy-mhicy" (pronounced "sheeky meeky"). Being seven-year "tourists" in Austria we spent an afternoon in each and agreed; the authenticity we'd found in other locales across the country made the towns less appealing. Then again, we had the luxury of time.

TrophyWife writes it best. Only you can decide your itinerary.