Ironically, the world doesn't seem to revolve around CHARNAME that much - most people don't know/care that you're a Bhaalspawn and you don't necessarily save the world - it's not even your goal. When you save Baldur's Gate/Suldanessalar it's more beacuse you were in the neighbourhood and the villain you were chasing was there. Even the final showdown in the Throne of Blood with (A)Mellisan is beacuse of personal reasons rather than trying to save the world - which seems to be doing fine with or without you.
I wouldn't call Irenicus a Bond villain necessarily. He's the one who said the famous "No, you'll warrant no villain's exposition from me" and only lost due to incompetence of his henchmen and circumstances beyond his control. He DID order Bodhi to immediately kill the PC. And he doesn't care about CHARNAME one bit - he cares about his powers and when he gets them, CHARNAME's existence stops being noteworthy. And he's understandable in a sense that you understand where his lust for vengeance is coming from. He traded love for power and lost both - vengeance is the only thing he has left now - irony being that his love would have forgiven him if he only said a word, but now he's unable to - classic Darth Vader tragic villain 101. It's not Tolstoy, but it works with the melodramatic, heavy on character drama story Bioware was telling.
Forgive my ad hominem - but you seem to be expecting a much different story than Baldur's Gate really is.
Also - if Thaos doesn't care about the Watcher and Watcher doesn't care about Thaos - then why is he in the game?
Thaos is for all intents and purposes a villain - or an antagonist if you want to get technical. Giving the player a reason to oppose him - which usually suggests some personal connection to the player character - would be a good start. Usual story has antagonist acting and hero reacting. Otherwise he's just a random NPC no. 34 in a stupid hat. On paper, you can make a story without a clear antagonist/conflict - you just need to be really careful not to bore and confuse the player so they won't ask "Why am I here?" You'll have to double down on player motivation for them to keep going through the story you crafted.
Do you think that the quest to solve the issue of your Awakening would sustain the game's main plot all by itself without any external opposing force like Thaos? It might work on technical level, but you'll still need some opposition, so the plot won't solve itself in 5 minutes. And "enemy within" stories are very tricky to write. From my experience, only Mask of the Betrayer and in some ways Torment pulled that type of story off correctly.
I am all for that types of ambitious stories - but there is this saying about falling from a high horse and breaking your neck.
And, yes the Watcher is not *required* to hate Thaos per se - but be consequent about it. Allow me to spare him or join his cause in the final showdown. Otherwise this doesn't work.
I'm not *expecting* a different story from BG2, it is what it is. I'm just setting out why I'm not particularly captivated by it, as perhaps a contrasting perspective to your own. Which of course to each their own, I'm definitely not intending this as a personal criticism or anything. I may not share your view, but I always find it interesting to hear how others experience things. But for me, when things get into "melodramatic" and "heavy on character drama", that's not really my preferred range.
In that regard, the story in BG1 is more compelling to me. Maybe not the whole 'dark destiny' thing that's obviously there as well. But there, the getting enmeshed in the political machinations, the iron situation, et cetera. It feels like you are much more part of a larger setting with things going on independent of your own actions; whereas in BG2, much of it feels more like a backdrop to you running after Irenicus (and Imoen).
Which is also why I like the PoE1 type of setup, where you see the same thing. The main character accidentally gets enmeshed in the whole thing, really. Thaos is in the game because he is a driving force behind what the Watcher has stumbled onto (including him being Watcherified), and therefore you as the Watcher get sucked into his wake. It doesn't fit the "hero vs villain" mould certainly, but that's not the only mould there is. A different mould it fits much better, is the "something's afoot, protagonist is trying to figure out what" mould. I mean, take Sherlock Holmes as an example: usually there wasn't really an antagonist around, that wasn't the draw of the stories. Or Heart of Darkness, entirely different mould of story yet again. These are undeniably compelling stories to many people as well, despite the absence of some personal hero-villain relationship or such.
Certainly I think Thaos is an important part of the story in PoE1 and it is better for his presence, but as I see it not because of that kind of classic hero-villain vibe. He embodies more the larger forces behind it, their agent. He is an active and variable component, in a way a dot on the horizon to focus the chase through the mystery. But there doesn't need to be a direct personal connection for that, and often in these kinds of stories that's only to the detriment; it's just so the author/director/whomever can set up some big, climactic, hideously contrived and unrealistic fight at the end or something (yes, by all means, let's lock blades and have a good discussion in the middle of a grand melee; that would totally happen in reality). If it's a cop movie, it's not enough that the bad guy has done Bad Things and the good guy is a grumpy detective dedicated to doing his job; there needs to be a backstory where the bad guy villainously set our hero cop's bunny rabbit on fire or whatever, and now. It. Is. Personal! *sigh*
Or somewhat more subtle variations thereof of course (though substitute 'assassin' for 'cop' and 'dog' for 'bunny rabbit' and you've essentially got the plot to John Wick there), and if done well that kind of plot device can certainly work. But I don't think it is by far a necessary component for good storytelling, and it is quite easy to go overboard with it.