Hey there!
I was much like you in that I played tzeentch daemons in 5th and shelved them. For me it was Dark Eldar and Grey Knights that sealed the deal. At first I was down in the dumps with the new dex but have since come around to loving the book. I think just about every Chaos God has viable builds IMO and Tzeentch is right up there. There is a lot of downtalk on certain Daemons, but I think that is just initial knee jerk speak.
IMO, Daemons are all about combos, working together in synergy. A little luck will also help, but if you don't like the random elements, then Daemons probably isn't for you.
Let's look at your models:
Fateweaver
Bloodthirster
Great Unclean One
Keeper of Secrets
Karanak
Changling / Tzeentch Herald
2x Demon Princes (1 w/ wings & 1 w/o wings)
7x Bloodcrushers
6x Flamers
10x Plaguebearers
10x Demonettes
10x Bloodletters
10x Flesh Hounds
15x Horrors
2x Soul Grinders
I think all of your models have places in lists and are very viable right now. Fateweaver is quite nice and acts as a big force multiplier, the reroll with the staff of tomorrow alone is huge. and 4d6 S5 Ap4 shots with precision strike and skyfire (when swooping) are very useful, not to mention prescience and the other powers he can get.
Bloodthirster is great - 2 greater gifts and send him at something to destroy.
GUO is also a beast, but slow, with ML3 and Biomancy he can be almost indestructible.
I'm using the Keeper in a Slaanesh and I'd say he is the weakest of the four, but can be great with the right rewards. Invisibility and Acquiescence are the bees knees.
I hear what you are saying about the troops, and most are complaining, but I think they are actually quite good. You can't compare 10 Pink Horrors to 10 PH from the new book, you need to compare 5 PH from the old vs 10 from the new as they are half the cost! There are lots of ways to run them, with 10 being great as they will get 2d6 shots until they die and with the reroll of 1's to saves are beasts behind an aegis or in ruins. You can also take a big squad with a herald with the grimoire and the 4+ inv power to give the squad a 2+ inv wiht rerolls to 1's. Add in the loci of conj and this unit is nuts.
Soul grinders with phlegm are great, just remember that it is ordnance and the harvester must snapfire when shooting both. Just about every mark is useful IMO for the grinder.
Probably the worst IMO from your stuff is the Bloodcrushers, but like you said, with Karnak they can scout and get move through cover. Again the the grimoire added to them, they have some options to become much more durable.
If I had to build a list with only your models I would probably choose:
Bloodthirster - 2 greater rewards
Fateweaver
Demon Prince with Wings MOT ML1 Armor, Greater reward, exalted reward (portalgryph or grimoire)
10 Flesh Hounds
10 Pink Horrors
10 Plaguebearers
Soul Grinder MON, Phlegm
Soul Grinder MON, Phlegm
That is 1595 and should be a solid core. Decent amount of just about everything. Light on troops, throwing in more (I'd deepstrike the daemonettes or bloodletters or keep them in reserve) the portalgryph can help with that.