Getting tons of UA miles is my main MS goal as well, but I go about it differently. I have 2 main MSing avenues:

1. Maximize the 5x category on my Ink, mostly with Staples VGCs (250k miles a year)

2. Use CFU at 1.5x to buy VGCs online through CB portals. This version is almost free though it accumulates miles more slowly than option 1.

You can always supplement this with any number of store bought VGCs but they will be more expensive. In the long run you are better off going exclusively for miles/transferable points if you value them at more than 2 cents.