Somehow I have difficulties in understanding how the common Commander Sharuum Combo works.

For reference:
It involves having a Disciple of the Vault and a Sharuum the Hegemon in play.
Then the player uses a copy effect like Sculpting Steel (copy effect has to be an artifact itself) on Sharuum.
The legend rule will lead to both Sharuums going to the graveyard and therefore triggering the Disciple while using the "When Sharuum enters the battlefield" triggered ability to bring back the Sharuum which will bring back the Sculpting Steel copying the Sharuum and so on....

My understanding (and quite sure a false understanding, that's why I am asking here) is:
When Sculpting Steel enters the Battlefield and becomes a copy of "real" Sharuum, the copy will put it's triggered ability ("When Sharuum the Hegemon enters the battlefield, you may return target artifact card from your graveyard to the battlefield.") on the stack.
But for doing so it will need a legal target. At this point however no Sharuum is in the graveyard yet because the legendary rule is a state based effect which will only happen whenever a player gets priority (rule 704.3).
My understanding is that a player will only get priority after the triggered ability is already on the stack and therefore Sharuum is not in the graveyard yet. So that the triggered ability cannot target Sharuuum.

Obviously I am wrong because this is a quite established Combo and should definitely work somehow.
Please help me clarifying where my understanding of the rules is messed up?

Thanks in Advance