What would you have done instead?
There's actually a lot of potential in the evil storyline, and it could be made far better with very minor adjustments. The potential lies in Mintharra.
Act 2 establishes that the artifact can extend its protection to other people, even brainwashed true souls like Mintharra. This is the way you separate her from the cult and get her on your side. The question is, why stop there? Have Mintharra or the Emperor propose that you subvert the cult and do, say, the following things:
1) One of the first things you see upon walking into Moonrise is Ketheric mistreating a bunch of goblins for failure. Wouldn't it be interesting, if you could abuse this conflict and turn the goblins against the cult? The goblins would stew on resentment for a while, and maybe when Balthazar captures you and intends to dissect you, they happen to be there, and now they're being mistreated by Balthazar. This is the boiling point, and you persuade them (or just mind control them, it's never established that's something Balthazar can do, but you certainly can) to save you from your cage. You do that instead of just breaking out, because in this version of the plot Balthazar is smart and takes your weapons, and also places you in a cage that can be broken by bare fists. Someone like Sazza could even be a companion - not someone with a deep arc, a lot of dialogue or a romance, just a little bit better than a hireling. To help you round out the team after the loss of Wyll and Karlach.
2) At another point, you can mind control a group of gnolls into eating their cultist handler. Wouldn't it be interesting if you could make them eat the other cultists too, when it's time for you to come to blows?
3) True soul Nere is someone you rescue in act 1. He's established as a rival to Balthazar. In act 2, he attacks him and gets offed off-screen. Wouldn't it be far more interesting if he survived? Amongst other errands Balthazar sends you on, he asks you to find Nere and kill him before he messes something up for the cult. Instead, you meet him and unbrainwash him. He thanks you, escapes and promises to inform the drow about the menace that is the Absolute.
4) In this version of the plot Roah Moonglow doesn't miraculously survive the slaughter of the goblin camp. She dies. She only survives and acts as a merchant on the evil path, and only offers you the act 3 deal if she's alive. If she's dead, a different Zhent takes her place and has zero intentions of having any conversations or negotiations with you, so no Zhentarim alliance.
5) In this version of the plot, Moonrise Towers staff went through a basic security training, and doesn't let a literally who, who could very well be a rogue true soul, walk around their base when there's no proof of their loyalty. A literal who is certainly not trusted enough to be given a mission that relates to Ketheric's source of immortality, that's only entrusted to someone who has helped the cult before.
In the end, Ketheric doesn't turn on you because you couldn't keep the artifact in your pants, but because he finds out that you keep poaching his staff, understands your motivation (to build a personal army) and decides to kill you before you become trouble.
And then, in the final battle in act 3, your splinter cult answers your call. The goblins charge in on your side, mind-controlled gnolls sate their hunger on the mind flayers, and hell, maybe there's even some drow involvement because some matriarch actually listened to Nere, why not.
Now, would this take some effort? Yes, I think it would. Would it take an overwhelming amount of effort? No, I somehow don't think so. But even these minor changes would make the evil playthrough immensely more satisfying.