Three nights in the Dolomites

We stayed here a few years ago. It is a unique place for sure. Highlights include the free spa area, the gourmet multi-course dinners and easy car parking.
Our room was spacious, immaculate and comfortable. I highly recommend it as a destination.

https://www.tripadvisor.com/Hotel_Review-g194841-d587525-Reviews-Hotel_Uhrerhof_Deur-Ortisei_Val_Gardena_Province_of_South_Tyrol_Trentino_Alto_Adige.html

Part of you problem is staying in the Cinque Terre, where often long transportation times are the result. Let's say you pick Sankt Ulrich, Hertz rental at the Bolzano Airport and a one-hour drive. Driving to Piazzale Roma in Venice from there figure 4 hours.
Train time from a CT village to Bolzano on an early train with 3 changes 6.5 hours.

But what rules this out is your needing to be in Venice the night before this cruise, or in a nearby town connected by train, like Padova , Verona or Lake Garda.

We loved staying in Ortisei and could walk everywhere, or get on a lift, from Hotel Grones. Beautiful spot. Terrific dining room too. Sorry, I can’t help with public transportation as we always rent a car. You could always take a day away from Dolomites and stay overnight on Lake Garda or Verona.

@N_Kingdom, Sorry if that wasn't clear. We do know that getting from Cinque Terre to anywhere in the Dolomites will be an all day trip, we've accounted for that in the travel plans. What we're stuck on is where to stay in the Dolomites that has a balance of public transportation and proximity to hiking. We may rent a car, but it will really depend on the cost of a rental for our time there vs the cost of public transport in the area.

Here is the bus schedule:

https://www.valgardena.it/dl/stchristina/pdf/350_15122019_12122020.pdf

Your train arrives at 13:48, 5 minute walk to the Bozen Busbahnhof along Via Renon [directly across on the 2 roundabouts]; bus at 14:28 [thereafter on the hour]. However you would be well advised to rent a car. The first bus back to Bozen Busbahnhof from Sankt Ulrich is at 7:34-8:36, train to Venice at 8:45-11:42; next one does not arrive in Venice until 13:33. Of course the way to beat that would be a last night at another town [I'd pick Padova], shortening a previous stay.

We arrange our cars by calling up the broker Autoeurope [having the use of a toll feee number]; get zero- deductible collision, which covers any damage to the body of the vehicle.

I'm making some assumptions here, but based on your outdoors interest, you do a bit of camping?

I don't know if this will be helpful or not, but we are planning something similar right now. In our case we are heading from Venice. We are bringing a tent with us, and I have found some really neat campsites. I figured what we save by camping can go towards the car. I was looking at public transport, but in the timeframe we have, it seemed too costly in time. We are going to have 4-5 days and then return the car in Trento. From there we will catch the train back to Rome for our flight home.

I've been looking at this campground in Falcade:
http://www.campingedenfalcade.com/estate/en/faq.html

Anyway, that may or may not be useful to you.

The Val Gardena is perfectly suited to car free travel and you can do everything including arriving from Bolzano by bus. You lodging will give you a pass good for the bus network once you arrive. The lifts base stations are mostly convenient to bus stops and the hiking is great.

The only issue is your need to get to a Venice by 1400. You could catch the 845 frecciarossa changing in Verona and arriving Venice at 1142. I would take a taxi from Ortisei (for example) to Bolzano for that early-ish train. We do it every trip both directions. Taxi takes 45 minutes and is door-to-door, a convenience we appreciate more each year. Cost is €90 each way. LMK if you need the name of a taxi driver. We know a dependable one,