The key event is the return of Morgoth. Sauron has a very healthy respect for Morgoth's power, and wants to see what the world will look like with Melkor ruling it. He's...by this time...a very committed/devoted follower. He doesn't have the zealotry of the balrogs, who are 'true believers' and fanatical. But he's made his choice, he's picked his side, and he's committed to seeing this through.
Considering who Morgoth *is*...following him down that path is going to drag Sauron firmly across the line. In the Season 3-4 timeframe.
The crisis at the end of the First Age (so, season 9 territory?) is that, suddenly, there is no Morgoth to follow. And Sauron does experience a moment of doubt, near-repentance, fear/awe at the power displayed by the hosts of the Valar...something like that.
Blind Guardian deals with the final conversation between Morgoth and Sauron in this way:
'The War of Wrath' - 1st voice is Sauron, 2nd is Morgoth
As you can see, they have painted Sauron as a devoted follower of Morgoth until the end. I don't think that's the exact note we want to hit. For one thing...Morgoth would take all of his followers down with him. He wouldn't magnanimously allow Sauron to escape while he goes down with the ship. Blind Guardian is setting up Morgoth as the protagonist of their concept album, and have to give him his moment to reflect on his failures - so it works for them. But we are going to have to show Sauron make the decision to flee/surrender on his own, not at Morgoth's command. And yet....'my servant you'll be for all time,' is the type of thing Morgoth would say to Sauron at some point. And it would be true, because Sauron *is* Morgoth's high priest as late as Numenor.
We have to keep in mind Gandalf's evaluation of Sauron's character at the end of the 3rd Age:
"For he is very wise, and weighs all things to a nicety in the scales of his malice. But the only measure that he knows is desire, desire for power; and so he judges all hearts."