This is how I build my minecart quantum stockpiles. I don't use tracks at all for quantum storage (I find setting up tracks and moving minecarts around not worth the effort in 99% of storage scenarios TBH), but the method is the same for you whether you use tracks or no.
1/ Build a Stop (b-C-S). Make sure it is dumping in the direction you want (d when first designating the construction site).
2/ Build a 1x1 destination stockpile (p) on the square where the stop will dump and then use (q) to make the settings store only what you want, with no bins or wheelbarrows.
For simple quantum stockpiling cases:
3/ Build a receiving stockpile (can be anywhere, but normally on the opposite side of the stop to the destination stockpile) of any size. The larger it is, the more dwarves will collect items, unless you set wheelbarrows for heavy items which limits you to three simultaneous collectors, or you can use mutiple receiving stockpiles.
4/ Construct a new hauling route (h), assign a vehicle (v), and make a new stop at your constructed stop (s). (ENTER) to define the stop, (ENTER) again to set the desired items to the same as your stockpiles, (x) to remove all existing conditions, (s) to make a stockpile link and choose the receiving stockpile to tell the minecart to take from it.
Done and done. Benefit from super efficient storage, more compact fortresses, shorter hauling routes, stocktaking at a glance with look (k), possibly higher FPS and a free set of steak knives. It is a bit fidly, and if you miss any step it doesn't work, but once you get it going it's a dream.