Err.. I've just noticed something:
The Signature Spell is subject to similar rules, but with two additional restrictions: It can only be cast while its owner's Oathbreaker is on the battlefield, and it always returns to the command zone upon resolution, overriding all other effects and rules, breaking the functionality of some abilities, such as buyback.
Source:
https://mtg.gamepedia.com/OathbreakerThis means aproach of the second sun goes back to your "command zone" and will cost 2 more mana, in stead of getting shuffled into your library. This means you can cut lots of your card draw for additional ramp.
/edit: and I've noticed another thing, the text of Aproach of the second Sun: "If this spell was cast from
your hand ...". So I'm not sure the deck will work this way.