Need help on planning : Amsterdam-Austria-Dolomites-Frankfurt

I made that route twice in two years. It was pretty tough to cover so many miles in 2 weeks and 3 weekends.
You might consider taking a train down to Cologne from Amsterdam so you can pickup a van in Germany--to avoid drop off fees. PM me and I can give you some tips, however you may be taking on a too many miles unless you're going to be there 4 or more weeks.

This itinerary takes you through some the most beautiful scenery I've ever seen anywhere, however.

If you need to tighten your proposed loop, I would consider cutting the Dolomites (despite the perception, these are not synonymous with the Alps in Italy, but rather refer only to a single subrange of Italy's vast Alpine territory). You'll hit plenty of amazing mountain scenery in Austria already. Whether or not the Dolomite range is more or less scenic is a matter of opinion, but going there will add time. Once you start to prioritize destinations, you can decide if it's worth the effort. I would also caution against allotting too little time to any one given Alpine destination. Alpine weather can be very fickle, and if you plan only a single day in one location without the benefit of a short-term weather forecast, you can easily lose that entire day to rain and overcast skies. Hence, negating all the effort to get there.