I really think the best way to deal with Astarion running off would be to reverse the order of the scenes.
Tav at the time of Astarion's scene doesn't know Karlach is about to burn up, that's meta knowledge. So Tav let's him run off, even though in that moment there's no reason not to chase after him.
It would be nice if Wyll and Tav could have a "You got her, I got him" kind of moment if you've gotten Wyll to blade of Avernus status.
Thematically, it might make even more sense to have Astarion start to run off, but everyone contributes to getting him out of the sunlight. He's not alone anymore, ya? It would be nice to have a moment where the entire group supports him. He's got a support system now where he didn't have one before.
