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What are other daemonkin players using for AA, I just dont find drake vectors to be that reliable.

As a solo army, the Daemonkin are almost entirely devoid of any anti-air firepower at all. Stock Chaos Marines are already extremely limited, and with the loss of Havocs it's almost zero. This is then further ruined by the Host's inability to take Fortifications, so you can't even fall back on a Quad-Gun or Firestorm.

In-house, your only options of any note are Heldrakes and Forgefiends.
'Drakes bring a S7/8 Vector and can take the Hades, but being frank you take a 'Drake for the Baleflamer. 4 BS3 shots is simply a poor output for a flyer of that cost.
Forgefiends bring twice the firepower for 1/3 the accuracy and a whole lot more fragility. Of course they make better anti-tank but that's not the problem being concerned here.
Finally, a Bloodthrister has his Whip. Whoopee!

By far, the strongest option is to take an allied contingent of Chaos Marines and get some Hades Batteries, Havocs and Helblades into the mix.
A pair of Hades are like a Forgefiend except cheaper, tougher and more accurate against ground targets. Do fantastic work against Serpents and are generally a superior choice.
Havocs spam out 8 S7 shots for 115pts. Honestly the Rapier teams are so superior for such a small price increase, but they are better if you have issues with Imperial Armour books.
Finally, Helblades bring out a 3-shot, AP3, Twin-Linked Autocannon with Skyfire, and do so for a rock-bottom cost. If you are looking purely for anti-air, these give the best results.

part of me wants to ally with csm, but then the little voice inside my head is screaming at me to keep as much BftBG as possible in the army.

What fliers are you specifically having trouble with? You should be able to ignore most fliers and focus on amassing SKULLS.

Absolutely. you don't need to worry about flyers if the bulk of your army is locked in cc turn 2.

If you need to use a Fortifiation with icarus lascannon+quadgun and put bloodletters in it, as they have BS5.