This is one of the byproducts of having companions as "Origin" characters.
Nothing and no-one can care about Astarion or help guide him on a path to redemption, because if this happened naturally, it'd devalue the effect that is supposed to be created when Tav/Durge actively makes an effort to do so.
The game is built around how your custom character/played character can take an active part of companions lives, but this limits the interactivity between companions so that things are based off your own actions.
It'd be much better if "Origins" were completely separate characters (Much like Durge) and so they had completely unique interactions with companions and stories within a game.
As far as Astarion himself goes, his only possible thread to salvation, is if he remembers his past from before he was a vampire (Where presumably, he would have experienced friendship, love, kindness etc). Which could be possible if the tadpole helped resurface those memories. If he remembers this better version of himself, he might choose to actively seek it out again rather than focus entirely on power.
