For the loose darts, when will you actually use them? When will you not be able to reload the triad from the darts on the ground?
Relying on scavenging ammo is never
a sound strategy.
Do not expect to scavenge anything at all. Consider a dart a simulated firearm cartridge. Once you fire it, it is gone. Carry enough ammo to have optimal effectiveness given no recovery. If you can then recover something, you are ahead of the game. As with many things, make this a matter of being on track at worst and ahead of the game at best.
where are you going to put spent mags?
Remember that a mag is not worth getting tagged over. A dump pouch is definitely a good idea, but if there is ever a question of your safety over what you do with an empty mag, you should check your priorities.
I tend to play like Alabasterslim on youtube. If under pressure, the mag goes on the ground. I can almost always pick up and store the mags after the fighting stops.
-Fanny pack with socks.
I would reconsider that. If you are going to have socks, they must be accessible. Anything that needs a zipper or mechanical closure is probably out completely. Pockets and pouches are generally not a great option either. A velcro-based rig is optimal. I recommend a velcro belt.
Other than that, this is looking good. I am going to second Herbert West on replacing the oddball 12-round mag.
Something that really needs a guide of its own, is ammo. Watch out for ammo condition, continuously cull garbage ammo as you load/unload before and after missions (if flattened, out of round, torn, bent, or loose fitting in a stock barrel it is at end of life and you should never use it in hvz), never leave mags loaded after a mission is over, and do not buy any more blue Elite because the reliability of that dart is not the greatest. I recommend you switch to koosh, particularly when you get into upgrading guns; but if you need ammo off the shelf, use ZombieStrike Elite or universal suction darts.
Be sure when loading mags to set back the rubber tips from the front of the mag body, and load properly (push down the stack and slide in the next round from the end, don't just stuff it through the feed lips).
Practice mag changes. With a pump gun that is a more serious matter than usual.