I see – we are working on a change that I think would handle your case.
We are going to break the upgrades apart where the cost increases (going from 1->4 usually has one cost, then it jumps when going from 4->8), and show those as different upgrades that you can do. For Hero track it is 1->4 then 4->6, and for Myth the cost is constant across the whole range, so there is no need to break it up.
Then, 99 times out of 100, if you just put those upgrades in order from most upgrade per currency spent to least upgrade per currency spent, that’s what you want to do. Just start at the top of the list and do the upgrades until you run out of currency. If an upgrade high in the list takes a crest type you don’t have, go down the list until you find one that you can do (or just save up until next week).
A note on why this works out most of the time: say that you have a Myth track item at 1/6. All of the 5 upgrades that you could do on that item have the same cost. And now let’s say that it ranks as your most efficient upgrade. Thus, you would always want to do as many of those 5 upgrades as possible before spending currency on any other upgrade.
Even if there was some wacky edge case where doing 3 upgrades on that item and 2 upgrades on some other item scores slightly better, the difference would be so tiny that it is a complication not worth exploring.