I currently have armound 1400.  ive had up to 5k.  I purchase every one from market, and at times grind campaign for no reason other than to snag mystery.  I used to float around 2k-3.5k, which was comfortable.  

when you begin leveling food in dungeons, you neglect campaign, and your supply will suffer.  You get them from campaign.

when you get a steady supply of ancient/void/sacred from cb dl your dependence on mystery will diminish.

it sounds to me like you need to be farming campaign.

As your account advances, you'll get more and more chickens. Doom tower gives a bunch of them each month, and you'll start getting higher amounts of ancient shards. At some point you'll have literally every rare you need, so blue = chicken.

In the meantime, if you really want mystery shards to make fusions easier, I'd recommend to never feed champions for xp, not even commons. Feed only for ranking up. This cuts down a lot on shards consumed while training.