Yeah, to get a more satisfying ending you have to
defy Bhaal after the Orin fight. It does not mean you have to suddenly be nice - in a dialogue choice afterwards you can say that you finally can kill for your own pleasure instead of Bhaal's. Which makes you a pretty horrible person and a good match with Ascendant Astarion. You still get the "you are the Heroes of Baldur's Gate" speech, wich is funny considering you are a villain at heart and might already plan to drench all of Faerűn in blood. But at least you can have a talk with your LI afterwards.
I agree that the pro-Bhaal ending is very unsatisfying - why do I have to take over my companions, especially my LI? There still could have been room for a talk afterwards with them, even if your are a super villain.
Even the ending where you supposedly take over
the brain with the Emperor together, he will just say "In my name" and have you as thrall. There is no ending that lets you mutually rule over the brain with anyone else.
They're allegedly working on it, but it'll be a while. Depending on what they do, these things take a long time to write, animate, voice act.
