Hi
I've not visited the towns you mentioned but have recently returned from the Dolomites, which I found breathtakingly beautiful and the area so clean. I would reccommend a place called Brunico a lovely town or if you are walkers continuing up the valley into the national park.
We stayed in St Peters (near Brunico) and it was approx 4 hours to Venice from hear.
Enjoy yourselves

Bigger but with a really nice centre and spectacular mountain surround is Bozen / Bolzano about 3¼ hours by train.

Personally I'd go for a two centre stay rather than rush a day trip from Venice.

Also, note that the real Dolomites, the region called Alto Aldige in Italian is officially bi-lingual and in practise German speaking so everywhere has two place names, starting with Süd Tirol for the region. It's a long story......

As Steve, says the Dolomite area is bi-lingual with road signs being in both Italian and German. Also the style of houses changes to Alpine looking, with steeply sloping roofs, geraniums on the balconies and lots of wood.
You could stay overnight at Lake Garda on the way to the Dolomites to break up your journey, if you have time.