We seem to have different attitudes about weather. When I think about winter in the interior, "terrific" is most certainly not the word that comes to mind. More like "frigid". So I will mostly ignore that aspect of your trip; if it works for you, that's all that matters.
Many of your stops are as yet unvisited by me, so I'll only address some of them. I don't know anything about the history of the Habsburg Empire, so please forgive my focus on general tourist interest rather than the specialized history that may be calling to you.
Zurich is pleasant, but it's a business city and very expensive. If I were paying Swiss prices (which are exorbitant) I would choose a place with more charm. Lucerne? Berne? Berne has a lot of arcaded streets, so perhaps more pleasant in the winter.
Brixen/Bressanone is a nice small town with a very pretty historic district. It is quite small, and I think one full day there might pretty much wipe out the points of interest unless you perhaps are planning day trips (by bus/lift) up into the Dolomites? The larger city of Bolzano, which also has a lovely historic district and good access to the Dolomites, has a number of museums that would help keep you occupied, including the one housing Otzi, the Iceman. You can move between Bolzano and Brixen/Bressanone quite easily by train.
Hallstatt is a pretty little place, but 3 days might be a long time, especially in the winter. Lake towns are sometimes dead in the off-season, though that may not apply to Hallstatt.
Prague is stunning, but 7 days is a long stay unless you plan to do something special (like dig into historical archives??).
Sibiu is nice and can also be a base for day-trips to other attractive towns like Brasov and Sighisoara. Getting into and out of Romania is not a quick process. It looks like the overnight train to Zagreb takes over 17 hours. If your time is flexible, I suggest seeing more of Romania. It's not so easy to get there, so you're not likely to return soon, and it has a lot to offer. But of course, I'm thinking back to what it was like in the summertime.
3 days at Lake Bled in the winter...I don't know about that. Without a car or a side-trip by bus (and the latter may not run in the winter), I'd have been going nutso after one night there. It is beautiful, but it is a very, very small lake. As in you can walk around it in an hour or two--something like that. There are lots of attractive outdoor options in that part of Slovenia, but I don't know what's available in the winter. Incidentally, it's an easy bus trip from Ljubljana (though I don't know the winter schedule), so if you really just want to see the lake, you can add a day to Ljubljana and avoid a hotel change.
I understand about the 3-night minimum, but 3 nights in Opatija and 3 nights in Venice seems a bit odd.
If you haven't already done so, I suggest checking all your transportation legs to be sure they are reasonable. I use the Deutsche Bahn for checking schedules, though at the moment the English-language page is switching to German when the schedule is displayed.