Something that occurred to me randomly regarding Astarion, his dreams and the artifact, which could further deduct that the Dreamer is indeed the being locked inside the Githyanki box.

Unlike the rest of the party, Astarion's dreams are the complete opposite. Whereas we are being attracted by our Dreamer and invited to surrender to the experience in order to become greater, Astarion on the other hand is being pushed away because the Dreamer is tormenting him by playing against his insecurities through the illusion of Cazador, which is why he has such strong emotional outbursts against us when trying to comfort him and why he is always uneasy and unable to relax.

And one may wonder why Astarion particularly, but it is quite an easy dot to connect and it is not because he acts evil or selfish.

Following everything else connected so far in this thread, the Dreamer is locked inside the box and can only directly communicate with us if we are unconscious. Outside of dreams though it can only indirectly communicate by acting as a sentient box who knows who to trust and who not to. It also protects us, while at the same time alarming us of danger when other factions looking to exploit it are close to discovering it or close to harming us, such as in the scenarios with Astarion or Minthara while we are sleeping.

It wants to stay with us because we are the only ones not looking to exploit it, so it knows that through us the box may finally be unlocked and the Dreamer that freed the Githyanki thousands of years ago may finally be freed from its shackles. Which is why it is also holding the mindflayer parasite at bay, because if the parasite kills us, the Dreamer forever remains imprisoned.

So... why is it tormenting Astarion? Precisely because Astarion is a sleeper agent of Cazador, yet another force in the world looking to exploit the artifact. Astarion was turned into a vampire spawn which enslaved him eternally to Cazador's will, something that he cannot resist mentally no matter how much he tries. And because Cazador is so powerful, the Dreamer knows if Cazador ever discovers the artifact then he may enthrall Astarion with ease and take the box, which would once again cause the Dreamer to forever remain imprisoned.

So in order to avoid this fate and ensure its freedom, it torments Astarion excessively so he does not get any bright ideas about the box since he is trying to find any sort of advantage in order to resist Cazador, which is why he is the only one advocating to overuse the tadpole's power without limits because he is constantly afraid of him and cannot resist him.

As for what happens once the Dreamer is free. I am positive the tadpole can only be removed by the Dreamer, just as it managed to free the Githyanki so many thousands of years ago. I assume that once we finally unlock the box, which will probably be linked to also freeing Astarion from Cazador and freeing Shadowheart's memories which will reveal she is a Selune follower abducted and brainwashed by Shar, the Dreamer will come forth and we will be presented by two choices.

Either we get rid of the parasite and by doing so lose our power. Or merge ourselves and transcend as the Dreamer always intended for us by becoming its champions.

[Linked Image from bg3.wiki] Minthara's Compendium Of Issues