What to pick!

So how does this game actually works with upgrades and what is the best why to do so?
Let’s take for example shellbro- gives 2 , then the x2 appear when there are at least 4 and no more than 8 meaning it appears in 4,6,8.
But then the game doesn’t really tell us when the x2 appear on the upgraded shellbro or when it upgrades appear. So regardless I’m never sure what is the best value picks