Venice to Lake Garda to Dolomites....Need travel advice!

Bolzano is more a nearby city / gateway to the Dolomites.
Ortisei is a small resort town within the Dolomites
So really quite a difference

It sounds like you are taking public transportation but will be going to the Dolomites after the season so that would be my concern #1 ; I imagine Lake Garda also being slow at this time so feel for your rushed plan off season you would need a car.

With your limited time I would skip Bolzano and stay your 2 nights in the town of Ortisei at one of the many nice hotels there.

Lake Garda I have only driven past so cannot comment on but since you have point A and point C defined I would choose a place on the lake to be Point B that was somewhere not too far of route A-C ; ideally renting a car leaving Venice and returning said car before/as you are entering Milan or at the Milan airport if flying directly out without going to the city.

Skip Lago di Garda. Unless you want to make a last-minute decision based on weather. If it is unseasonably nice, you could go. But it really is a good place to have a car. Otherwise not too much to do and I am pretty sure there is no/limited ferry service at Limone after mid-October.

Bolzano is a safer bet for all three nights. If it is nice, you can take the bus to the Val Gardena in about an hour. It is off-season, though, and a lot of lifts are shut-down/close-to-shut-down depending on the exact dates you are there.

Here is the schedule for the Alpe di Siusi lifts https://www.valgardena.it/dl/stchristina/Lifts_Summer2017_no_valgardena_card.pdf

Here is the schedule for the Val Gardena lifts https://www.valgardena.it/dl/stchristina/Lifts_Summer2017.pdf

Note dates are European style: DD/MM/YY.

Another option from Bolzano is to explore the Renon Plateau. Although you would not be precisely in the mountains, there are some views and light hiking.

EDIT: I love Ortisei, but the weather is such a gamble in October you might be disappointed. That's why I am recommending Bolzano since you are thinking of staying there anyway.

far too much for 4 days. Lake Garda would take a week! The Dolomites would take months. But by mid October the mountain passes start to close so some places are out of reach.

First problem, the lake only has railway stations at a couple of places on the southern tip. So unless you waste limited time hanging around for a bus to go further you are restricted in where you stay. That said they are well placed for getting to Milan - even close enough for afternoon/ evening flights (wouldn't try for morning). Sirmione is really only a small village at the end of a thin sliver of land which is almost totally built upon, I'm not sure it's worth spending time doing the bus transfer. Peschiera would make best use of the time, the views are similar. But personally I'd just do 3 mights in Bozen (Bolzano).

Bozen is a really nice town and transport hub, it's easy to reach into various valleys. It gives you flexiblity with the weather as it can vary, if your first choice hike had a bad forecast there may be somewhere with better. It's the capital of the german speaking area of Südtirol. The italian speaking areas are better served from Trento or even somewhere in the north of the Venice region.

I would rent a car for what you intend to do. You seem to have only 3 nights in total before having to go to Milan. It's a short time to see both the Dolomites and the lake.
To have a glimpse of both, I'd drive from Venice to Ortisei via Cortina. Spend two nights in Ortisei. Then drive down to Lake Garda. Sleep one night in a town on the north lake (Malcesine, Riva, Torbole, Limone, etc). Then drive south along the lake the next day on my way to Milan, maybe stop for a short visit in Sirmione on the way there.
I've spent time at the lake and the Dolomites a couple of times in October in the past. If it doesn't rain it's nice. It's not busy, so accommodations should be easy to find anywhere. But October weather can be iffy. If it rains, there isn't much to do. I guess you have to take your chances.

We did a similar route last summer using public transportation, but stayed in Verona instead of Lake Garda. Verona has a good train hub with an easy train ride from Venice as well as up to Bolzano. From Verona, you can go to Sirmione as a day trip if the weather is nice.

Thanks everyone for the advice! Much appreciated!