Interesting gifs...
IMHO I believe that Tolkien's concept for Sauron is most likely Satan from Paradise Lost. Tolkein was a devout Christian and for British people, particularly the educated Paradise Lost was a definitive depiction of what their philosophical understanding of Satan or unrestrained evil was.
Satan was meant to be charismatic demagogue and likeable person by his followers but unrestrained in his desires, and rejection of the concept of evil- in which he becomes more and more unrestrained until in the end he physically becomes more snakelike than the glorious archangel he was.
The Christian idea is that Satan was originally a servant of God but found his lack of freedom oppressive and questions' God's authority & his rules. By doing so he questions the very concept of morality- and rejects it. Instead craves unlimited freedom to do whatever he wants. This is in the road to hell.
IIRC from Simarillion Sauron is described by Tolkein a bit like that, but it's history/lore notes so we don't know for sure.
I'm sure that Tolkein scholars already pinpointed where the character originated from.
If they go this route with ROP S2 then Charlie has to play a challenging role- not everyone can be charismatic or even learn how to be.
Come to think of it the idea of a gradual increase in evil is something that would work well in ROP.