I agree on the sagging comment. I am using 3/4" EMT for my vertical supports. I have 2 sections (20' tall) connected with the coupling, which is a wimpy connection, so I slipped a 1/2" EMT section into the 3/4" to help strengthen that joint. Maybe doing something like that would help your sagging? If you could add a center row of support to help hold up the middle that would be even better.
I'm in my 2nd year of growing and it seems every week I wish I did something else to strengthen my trellis. Last year wasn't too bad but this year they are growing like crazy! There are already close to 20' tall and just starting to flower.
You're off to a good start...Good luck!