Happy to have Simmons as a BBEG - Counterpart was one of my favorite series, I was really annoyed when it didn't get renewed. Guy has an amazing emotional range.
If we use the list provided Irenicus would be a 5 with a touch of 4 & 3. But Irenicus was really Lucifer meets Icarus meets modern evil - the greatest of all elvish wizards meets the hard limits of his power* so his 'sister' convinces him to break the rules of universe and become a god. He fails and,at this point he's Lucifer-Icarus but then he's punished with the loss of his soul which adds a layer of modern evil to the black forest cake of his badness. He becomes unfeeling and clinical - essentially a lich in a living body.
While we all worship David Warner I agree with David Gaider on his performance: really good but he flubbed some of his lines. Gaider wrote the lines where Elliseme is released and he said Warner didn't quite get what Gaider was going for - his voice is supposed to communicate great pathos but it doesn't. And there are other flaws: Irenicus was supposed speak without feeling until he takes charname's soul and only then would his voice be full of passion.
I don't think Simmons will make the same mistake - in couterpoint he was able to change from ruthless and cynical to hapless and pitiable in an instant and I think he'll do the same in the game. It looks like we will able to pity Ketheric in the way we were supposed to pity Irenicus. Looks like Ketheric will be 5 with a touch of 4 and 7 - the absolute is the real end boss (that we'll see in BG4 or DLC 3)
It's a great choice - if you haven't seen counterpart you should watch it. Underrated show.
* I never played with a group that took this seriously but in 1e elven wizards could not go beyond lvl 10
