A city built at the scale of an empire and now used, very calmly, as a place to look at drawings the size of a hand.
One entry, one queue, and a long warm sit afterwards. That is the whole shape of the day.
Check what is actually out before you book anything. Works on paper are shown in rotation and rested in between, and the drawings people cross a continent for are frequently in a drawer. I have had both kinds of morning here.
Go at opening and go on a weekday. It will still be busy — it is always busy — but the first hour is the only hour in which you can stand in front of a small sheet for as long as it deserves without apologising to the person behind you.
The city is walkable at the centre and the trams are pleasant rather than necessary. Budget an afternoon of doing nothing afterwards; looking hard at drawings is more tiring than walking a house, and Vienna has never been short of somewhere warm to sit.