You could also simply switch the control to Wyll, wouldn't be the first time the game does that.
As for the rest, using ascension to achieve sunlight protection in order to save Karlach, is equally metagaming because the ritual is not about fixing Astarion's sunlight problem, this is only ever portrayed as a nice bonus. And I am saying that as someone who's first ever run was with Astarion Origin. But I also know that for a lot of people the draw and sole point of the ritual is magical sunscreen, so there's no reason discussing this any further.
It's pointless for the situation mentioned here anyway, which also applies for Avatar-Lae'zel or someone leaving with romanced Lae'zel (including Avatar Spawn Astarion). It has nothing to do with tragedy and everything with the line up of the endgame cut scenes. Before patch 5, Avatar-Gale also couldn't save Karlach because his dock scene ended when he gazes over the Chiontar, pondering the crown. With patch 5 they slapped on the Karlach ending, giving him a crippled dock scene (the whole middle portion is still missing) in what seems like a quick fix. I hope that while adding evil endings, and adding new lines for Companion-Astarion's departure, the whole scene got a little polish.