We were there during spring break last year (march 29-April 3) and it wasn't super busy during the week. This is coming from someone who has only done off season in the past. As long as you arrive early, grab fast passes and go knowing it will be crowded you will have a nice trip. Definately read hydro guy's tips!
Also if you stay to park close it tends to thin out a ton after fireworks. We were litterally walking onto most fantasyland rides, walked on the nemo submarine ride, bobsleds were a 5 min wait and space 10 min. I would avoid the 19th since it is a friday and the locals may come in after work/school. During the day I remember Space having a 60 min wait at one point. We actually ended up with left over fast passes because we didn't need them!
If you need any more help feel free to PM me