Should be a pleasant trip overall as long as you don't have to rush. One slight problem is that you cannot leave Duluth too early since it is cold, but Superior never really warms up that much anyway, guess you just need to bundle up well. You will need warm clothes in the lower St Lawrence as well.
If you can make the time, spend some time on the south coast of Newfoundland
. It is an incredible cruising ground. You need to be confident enough in yourself and the boat to operate independently there - not many people around. Also the Magdalen Islands Quebec
City are well worth a stop.
The locks downbound are easy - two people can do it, although three makes it easy. You want a good boat hook and gloves. Certainly use the tides in the St Lawrence below Three Rivers - you will go really fast. We came up the river and were doing more than 10 knots over the ground, upstream at one point.