I suggest you don't plan to do this yourself and take a coach/cruise/coach tour.
That is 8 hours of driving without the stops and cruise and lunch. Either take a coach or stay 2 nights in Te Anau. If between May and early November best not to self drive at all. There is very little difference between the cruise companies, just smaller boats or bigger boats.
Milford Sound - last 30km of the road is narrower, windier, steeper. Scenery outstanding. Everyone suggests catching the bus (from Te Anau would have cost $80ea return) to ensure you don’t miss the view and as the road is challenging in places. However not a big problem for us today. Fine weather, plenty of time (4hrs comfortably from Qtown), and we were ahead of the traffic. Had time for coffee at the cafe before boarding the cruise. We booked for the 2 hr afternoon nature cruise with RealJourneys.
Note there is an at least 10 min walk from nearest parking to the cruise terminal. No petrol/diesel beyond TeAnau. No phone service. Just beauty and many bus loads of tourists (we went on a public holiday in October, lovely weather 14-20C). Cruise terrific!! Very generous picnic lunch if ordered.
Lucky you JT No avalanches today to delay your trip home or Hundreds of dollars to fly you and get your rental vehicle out once the road is open again.
Over 8hrs driving there and back to QTN via devils staircase when you are tired in the afternoon (where there has been two accidents in the last couple of weeks). Glad you and the other road uses you encountered on the trip arrived safely.
Not a good idea to do this trip in a day unless you leave it to the professional coach drivers and people who are paid to get you down and back safely.
Yes the whole country has had 3 days of near perfect weather for a long weekend. However heavy rain is forecast for Fiordland tomorrow (not unusual). In the first week of November, 2008, I planned to drive my sister (we were staying at Manapouri) to Te Anau. I still lived in Australia at that time. I continued driving the rental car although snowing but decided to turn round at the Divide as too risky to continue. We had lunch back in Te Anau watching the snow pelt down. The next day we did a glorious boat and bus tour to Doubtful Sound, blue sky but the trees covered in snow, I have some stunning photos. My sister had to wait until 2011 when (now living in NZ with my own car) I drove her and her husband fromTe Anau to Milford for the day. You can be lucky in October, watch the forecasts but I would never drive (even in my own car) from Queenstown to Milford and back in the one day.