the 15 point mark is exactly the amount you need.
from your story I think you only use the first row to start your engine and then move up to second row.
but there are 5 paths to follow, each with add and dis-advances.
the fun thing is that all strategies will reach 15 mark, around the same time, the time can differ depends on how the players have played and if they blocked each other or not.
back to the basic:
cards which need more of the same will cost less then if you need more diffrent kinds.
for example:
1 white, 2 green, 1 black = 1 red card
3 green = 1 red card
4 green = 1 red card + victory point.
this same applies to the top level:
7 green = 4-5 points
7 green + 3 blue = 5 points
but keep in mind you may have 3 jokers, so 7 green is doable and is the most cost efficient way to go.
but can be counter, by players keeping 1-2 green coins in there pockets to block you.
if there isnt almost any green coins needed, you can probley get them for cheap.
and you can steel some cheap level 1 green cards to assist you.
the person who mostly buys level 1 cards, can easy get the bonus cards of +3 vp.
midrange
you focus on getting as fast as possible to mid range cards, and will only buy level 1 to assit you
and then you have 2 hubrides, early - midrange and midrange late range with some early range.