It depends on your interests!

That said, it strikes me as ambitious. For example, I personally would find just 3 nights in Prague and just 2 in Vienna very frustrating. For one thing, you will lose a chunk of time in transit between those cities. Too, I love museums, and Vienna has some of my favorites - but maybe that's not something you plan to do.

The number of nights you are planning in various places could work for a fast paced journey if your goal is to see only highlights, but it seems to me that you'll spend a LOT of time traveling relative to the time you'll actually spend anywhere and you'll be skipping a lot (particularly in Croatia).

But you'll see some wonderful things (and I have seen all of them except Hallstatt)! Hope that helps.

Why not stay in Hallstatt instead of just visit it en route. It is lovely to walk around in the morning and evenings after the day trippers have left. Some very nice restaurants also.

I think you are cramming too many countries in and shortchanging all of them as a result.

I would recommend doing Slovenia and Croatia in that time frame or Austria and CZ. Slovenia offers the Julian Alps, canyoning, fab hiking the Soca River valley, the Logarska Dolina, and the Istrian peninsula which takes you down to Croatia and Plivitce, Dubrovnik, Hvar and Korcula and various other islands.

OR

Austria and visit Vienna, Hallstat, Salzburg, Grossglockner pass, Innsbruck (not in that order) heading up to CZ and Prague, Cesky Krumlov, Karlovy Vary, Terezin and Kutna Hora.