I'm not going to advise you about daemon summoning as I dislike it as a strategy. It is too time consuming and requires too much book keeping.
You are spending a lot of the points on the heralds, but not really getting much in return. I'd only run 1 herald and save some points.
How competitive are you being?
Because everything now scores you could make a list like this if you want to pull some cheese:
Herald of nurgle - Greater or Exalted locus
Nurglings x3
Nurglings x3
Soul Grinder - Slaanesh - Torrent
Soul Grinder - Slaanesh - Torrent
2 AV13 walkers at 500 points is very difficult to deal with. You could always remove the locus from the warlord, and have one or both of them as nurgle grinder with phlegm cannon if you want to keep it themed.
Might not be a bad idea, the battlecannons will be more worth their points if you are only going to be playing marines.