To the best of my recollection, once you get north of Stoneham Rte 28 through Reading to Andover isn't bad, Rte 133 through Andover is decent, and 125 is OK. Stay on 125 through Haverhill, DO NOT GO THE WAY GOOGLE MAPS RECOMMEND THROUGH HAVERHILL!!!! Not bike (nor people) friendly area.
125 through Plaistow is congested, it's a retail district, but North of Walmart the road is not bad.
121A and 111A aren't horrible, 107 is OK.
Some of these roads I've ridden, but 15 years ago when I lived in the area.
Now living near Manchester, NH I'm not riding as much as I used to, hopefully someone else will give more current information south of Andover.