I am not convinced of being too late. I think Chat GPT is correct on this, as long as the player has just started Act 3, possibly haven't been in the city yet. However, Astarion is unique in that his personality may change dramatically, which may lead to something hard to predict. Good luck.
I think the only romances you can do in act 3 are Halsin and good recruitment Minthara. The rest already had important scenes in the first two acts.
