Well, first of all, do you have an HQ for any of these guys? A Herald of Slaanesh, or something? If you have a big squad to benefit from her buff, I tend to find that a HoS with Exalted Locus of Beguilement, a Greater Reward, and a Steed of Slaanesh is going to wreck some serious face.
I think a real danger here is how damn fragile each of these units are--you don't have any defensive buffs like Grimoire or anything, let alone Telepathy levels towards Invisibility or something. T3 models with a 5+ save disappear fast--especially in minimum squad sizes of 5 or 10. The Nurgle units are marginally tougher, with T4 and Shrouded, especially if they're hiding in a ruin, but... they're only half as fast as the Slaaneshi units.
I'm also seeing something of an over-purchasing of upgrades, honestly. One phase of combat for a banner doesn't quite justify it, in my eyes, most of the time: are you also DSing some unit in on that banner? That double stacking of utility is what makes them worthwhile, to my eyes... but I don't know why you'd make sure to buy so damn many banners. And while greater etherblades are great, those champion upgrades make up about 180 points of your list--that could be a lot more daemonic footsoldiers, if you wanted to bulk up the squad sizes a bit...
Ironically enough, the unit that I think could most use a banner doesn't have one. The problem with nurgle daemons, Plaguebearers especially, is how damn slow they are--so I kinda like DSing them in. Now, when they come in they can't run to spread out, so beware cover-ignoring templates and blasts--but dropping a unit of Plaguebearers into a ruin for a 2+ cover save with no scatter from nearby Plague Drones with a banner... mmm! Saved yourself several turns of slogging up the board and getting shot to pieces!
Not that you need to buy the Drones every single upgrade. For one thing, you're only allowed to buy either rot probiscus or venom sting...
So the question is what are you doing with the PB units? Plagueswords are great for whittling down vehicles or killing anything with a Toughness value and a less-than-2+ armor, but... they're a melee unit with Slow & Purposeful. If you're not DSing them, they're basically only good for camping on a home objective--a job that Nurglings do better, with more wounds, for half the price of the squad. Perhaps take one unit of 15 or so PBs, and one unit of 3 Nurgling swarms? It would cost just about the same number of points, and would let you DS the larger unit of PBs in on the Drones, while letting the cheaper little unit hold your home objective.
I'd honestly probably shoot for one larger unit of Seekers than 2 tiny ones. One of those tiny ones will disappear the first time so much as any light infantry fire wisps in its direction.
I... honestly haven't played with Seeker Chariots. They could be excellent, for all I know, but... they looks like such fragile glass cannons, on paper. Really focusing on defensive buffs like Grimoire and Invisibility would probably make them function well, I know, I know--it's just like a normal Slaaneshi glass cannon list, for me...
Ahem. What I might suggest with similar minis at 1k points:
110 - HoS w/ Greater Reward, Beguilement, Steed of Slaanesh (joins Daemonettes--may Outflank w/ Acute Senses if desired, thanks to the Steed, and gives a lot of Beguilement rerolls)
75 - HoN w/ Exalted Reward (swap for Grimoire) (joins Nurglings)
171 - 19 Plaguebearers
60 - 4 Nurgling Swarms
180 - 20 Daemonettes
211 - 3 Plague Drones w/ Venom Sting, Death's Heads, Plaguebringer w/ Lesser Reward, Plague Banner
193 - 14 Seekers
The 19 Plaguebearers lets you kitbash a HoN from the set; the 14 Seekers lets you make a HoS from the third box of Seekers. I figure you can get a 4th base of nurglings with a judicious spread of the Nurglings in the Nurgling kit and in the Plaguebearer boxes...
The Plague Drones are a kinda hypothetical combo, there, for the Plague Banner plus the Venom Sting, which is... certainly not the most reliable, with no AP and WS3, but 3+ poisoned upgraded to 2+ poisoned is less nice than Instant Death 4+ poisoned upgraded to Poisoned.
And the Plaguebringer--well, he can make an AP2 IDing attack with 2+ Poisoned when the Banner pops, so provided that he can hit the target... I figure that little combo makes the Lesser Reward fine, since +1S for the Greater is not so useful when 2+ Poisoned is also in the cards, for at least one phase.
You could make the unit a lot less expensive by taking a mere Icon of Chaos, no squad leader upgrade, and no venom sting--but where's the fun in another 55 points to flaunt freely? Just, say, 2 Mastery Levels... or a handful of Seekers/Daemonettes more.
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!