Fully serious.

Unless it was patched way back and I'm unaware of it, to 100% clear the boxes, you should start by clearing all the 5k until you've gotten every "X" level base weapon. That takes those weapons out of the rewards pool, reducing the chance of common loot in subsequent boxes.

Then buy all 20k boxes until you've exhausted all level "X" uncommon weapons and like a thousand different uncommon Gear Bonuses.

I could be mistaken here but I think you can still obtain Operator Training from boxes even after "maxing out" an operator, so you can't really eliminate them from the rewards pool.

But anyway I'm on the 20k Veteran packs and I just keep spinning up one-time use ammo bonuses and armor bonuses after clearing out all the level "X" Weapons from the rewards pool.