That is the problem. this isThere is no 'Downtime by level'. It's not level dependent (however the rarity of items you can create is, and gold is another factor).
Artificers can simply create a common item in a single day, costing them only 50gp to do so. An uncommon item takes 5 days and 250gp (going by the DMG). Halve that for a consumable.
They can pretty easily outfit the party with +1 weapons pretty early on (3rd level) for not much gold at all (4 PCs and 4 magic weapons = 1000gp)
Havent got XgtE here, but I presume (once they have a formula for an item) they can really churn those suckers out making a tidy profit.
Rarity is a useless metric for this as for example a +1 greatsword & an amulet against detection & location are both the exact same uncommon rarity &