What to spend crafting supplies on?

The best thing you could do is probably wait for an upcoming Heirloom event. If you don't mind spending money, the wisest thing to do would be to use your Crafting metals on all 12 epic grade items as they're cheaper (800 crafting metal each). This means you'll only have to spend 9,600 of your crafting metals meaning that you can still presumably buy one legendary item. This means that you would only have to buy 7700 Apex coins for the remaining 11 packs and easily get the newest heirloom for the cheapest price possible.