There has not been a multiple vote at a national convention since John F. Kennedy tried to get the vice presidential nomination to run with Stevenson in 1956.
Why he wanted it is beyond me. Stevenson had no chance at beating Eisenhower, and vice presidential nominees on the losing ticket have all been shoveled to the dust bin of history, except one. Franklin D. Roosevelt ran with James Cox in 1920.
That last presidential nomination that went beyond the first ballot was at the Democratic convention in 1952.
As for this time, Michelle Obama would be a nightmare. She her views are not much different from AOC and her gender, race and association with her husband would have more than half of the stupid people voting for her without even listening to what she would do to them.