To my eyes/preferences you've taken a bit much aircraft and AA, do you really need all those Thunderbolts and Hydras? I'd consider replacing a formation of Thunderbolts with either some Slaanesh Demon Knights or a Subjugator.
You really must have a Demagogue in the formation with the Chaos Altar to be able to summon (and very usefully retain on the board) the Greater Demon. If the Altar is taken out you'll loose the ability to keep the GD down and reliably summon it, so you may want to consider a second Chaos Altar + Demagogue in another formation.
As Hena has said I'd try to take more lesser demons than the 4 you have.
I reckon adding Leman Russes to a Mechanised formation is a bad idea as it slows them down and gives them two things they want to do; shoot and assault. Does make them tougher from AT fire, but I reckon the benefits outway the negatives and you're better spending the points elsewhere - perhaps on another Coven or Plague Zombie formation to up your activations.
Take Chaos Spawn! Your BTS formation needs one at the very least and I'd think about it for your other priceier covens. They're good in combat but most useful for being both tough (3+ save and invulnerable) and Fearless. If the rest of the formation gets wipes out through combat resolution or blast markers once broken they survive. Which is useful for keeping activations up, preventing BTS loss and limiting VPs opponents get if it goes to that. They're 25pts, so at first glance costlier than a Big Mutant, but a Spawn is an addition to a formation rather than a replacement of a cultist/mutant so actually cheaper. Plus they're just very chaosy and cool to convert and model
What numbers of mutants to cultists to big mutants do you plan to take in your Covens or think is optimum? This is something I'm musing on myself. I've used all cultists with 4 Fire Support, which can put out 20 shots at 30cm, pretty shooty for 300 points, though you really need to keep them in terrain or they'll get slaughtered in short order. Or mutants are good in combat and have a save but you sacrifice any kind of ranged shooting. So far I've gone for either cultists or mutants, with a few Big Mutants with my BTS/Supreme Commander/Blitz Guard formation to make it tougher and pricier, but am curious as to whether anyone mixes them more.
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