Hi Mark
That makes sense, as transfers on planes are much worse than transfer on trains
As you'll have a car, it does open up the option of a road trip, and for this agriturismo.it can be ideal, as they're usually very nice farmstays in lovely countryside. Not 'slow' but it can be a nice way to amble down from the lakes via Piemonte region, ending up at CT.
There may be options for parking the car, but for visiting CT, it's a waste having it sat there when you'll be traversing the villages on foot, by boat or train.
Toying with re-ordering...
If starting in Milano meeting your friends, you could afterwards get the fast train from Milano Centrale to Genova, then the coastal train to CT.
Then look to hire a car in La Spezia, Levanto, or anywhere on that direct train line (including Genova). Drive up through Piemonte - either to a single base or use the road trip idea. Keep the car for the lakes to really open them up, dropping it off somewhere convenient to get to Milano Centrale for the fast trains to Venezia. It's doubling back, but might flow better as it keeps the driving legs together.
Given the interest in wine, I think the Langhe does shout out as long as you've got a keen interest in Nebbiolo. We've enjoyed many stays there and a car is near essential. We've stayed in Treiso, Corneliano d'Alba, Santa Vittoria d'alba (especially recommended - val di spinso, which has spacious apartments and a pool in lovely grounds), Serralunga d'Alba, Monforte d'Alba and between Barolo and La Morra. The walking here is lovely. We've also stayed in Ghemme, and on the outskirts of Asti as well if they appeal. The food is wonderful and I'll especially put in a good word for little Treiso, with all 4 places to eat very good in their own way. Conversely I feel Barolo (and to a lesser degree Barbaresco) are showing signs of tourism taking away some of the charm. The other villages retaining it much better.
The walking is also excellent there, and shared inheritances / resultant shared ownership means almost every vineyard retains open access, not just for the vineyard owners, but also us mere individuals. The paths are good and maintained. That can be a very special experience.