Sorry for taking a while to get to this, PM and all.
Thousand Sons are very good in their niches of "mid-range killing MEq" and "taking heavy duty punishment and still standing," but they come with the pretty hefty downsides of high cost, being poor in close combat, Slow and Purposeful... and dying just as easily as a marine 10 points cheaper to light arms fire.
That's not to try to persuade you not to take them: they're very cool and characterful models, after all, and you're the one saying competitiveness isn't your top priority. Do be careful with them on the battlefield, though: they need to be pretty well supported by other units like your Helbrute there, and I'd honestly field only one full sized squad or two minimum-sized squads. There's nothing wrong with taking a dirt-cheap squad of unmarked cultists in a Tzeentch list...
With 7e's changes to the psychic phase, a big block of Horrors with a Herald with an Exalted Locus... isn't that great, anymore. It was kind of borderline in 6e, when there was chances to perils and enemy anti-psychic denial abilities... but now, you have to dump an awful lot more power dice into the unit to (even probably be able to) get the S6 Flickering Flames off on different levels, and you can only cast the power once per unit (so the joined Herald can't cast his own Flickering Flames using his Locus, too)--honestly, I think that ship has sailed.
Horrors aren't half bad at hanging back to defend objectives and acting as Warp Charge banks to fuel your better psykers. They can even roll on Daemonology for a chance at a good power like Cursed Earth, coupled then with the free Flickering Flames and Summoning Primaris Powers... they're worth the points, certainly. But I don't think trying to make them into a strong shooty unit is going to work out too well, honestly. Consider dropping that Herald entirely and bringing the squad down to 11 members to free up some points.
That Lord of Change is looking pretty good, though I'd grab a second Greater Reward.
I'd try to fit that Tzeentchi Soul Grinder back in--pretty easily doable if you've cut out the Herald, 8ish Horrors, and a few 1ksons, I think (if not the 1ksons, since you want to keep them all, well, perhaps a Screamer and downgrading the second tl-las/las pred to auto/las?). It would mesh with the Preds and Helbrute well, to my mind. Consider giving it Warp Gaze (and casting Prescience from a Herald/LoC on it) or Baleful Torrent, since you already have a decent amount of AP3 shooting in the 1ksons... better to cover more bases with some ignores cover or something...
A problem here: what units are you going to be Infiltrating with Ahriman's Warlord Trait? The 1ksons don't want to get too close to the enemy, though they might work, but... there's no real infantry here that needs to close the gap fast to be effective. Consider taking a Tzeentchi Sorc who can grab a spell familiar--sure, he won't be ML4, but he'll be getting his powers off much more often. Then he can get a SoC for a 3++ (with MoT) or get Termi Armor for a 2+/4++: probably more survivable than Ahriman, even with a wound fewer, and can take a force axe or something. Ahriman is a lot of fun, though, so feel free to stick to your guns with him. If you do swap Ahriman out to save a lot of points, you can grab another Mastery Level on the disc Herald, if you like, among other things...
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!