I think you'll do well just with point-to-point tickets. Most of your travel is in Italy, where rail fares are comparatively low. You can save a lot of money by early purchase of train-specific non-refundable/non-changeable tickets if you are prepared to set you itinerary in advance.
Ortisei is reached by bus. I believe the nearest major train station is Bolzano.
Italian fares can be checked on the TrenItalia website. You'll need to use the Italian city/station names:
Roma ( Tutte Le Stazione )
Firenze ( Tutte Le Stazione )
Venezia S. Lucia
Bolzano Bozen
I'll leave the trip from Bolzano up to Garmisch for someone else to deal with. I think it's an Austrian train.