lets say i bought 20k qi, would it raise me to the appropriate belt level or would it only reflect for future shop purchases?

Buying QI has the same effect of playing that amount of games afaik. So you should get both higher belt and be able to purchase higher tier items.

If you gonna buy 20,000 qi then you gonna get custom belt+ and , you can purchase high tier items such as Void , Pure , Demon and etc.