God Gale does cure Unascended Astarion and Karlach, video inside a spoiler if you want to see it:
Ascended Astarion can't cure Karlach, he will break up with her after attempting to do that unless you go to the final boss without long resting right after completing his storyline.
Personally, I really like Ascended Astarion with Minthara, because she is the only companion in the game who approves of his ascension and encourages him to seize the power. She also wants to dominate the final boss, and both her and romanced Ascended Astarion have new animations (in patch 6) when you decide to do so. In the epilogue, even if you go for the good ending they're still portrayed as a wicked power couple, working in tandem to become shadowy rulers of Baldur's Gate. Another thing they can bond over is knowing what it's like to live in the shadows and finally having one loyal person they can trust after a lifetime of hardships. Sadly, Larian keeps neglecting Minthara and she doesn't have much content in the game.
Astarion and Lae'zel are also a very interesting couple, seeing how they get together during the party if you don't sleep with either of them. She also admits somewhere that she's not opposed to the undead, considering she worships the Lich Queen. In the epilogue, if she leaves to fight in the Astral Plane alone she also has a unique line for Astarion. The last patch made it possible to leave Faerun with her, if you're not gith yourself, so it's also an extra way for Astarion to avoid sun damage if you don't ascend him. It's rather cool storywise too, since if you play as him she helps you free you of your master and then you can return the favour and help her fight for the liberation of her people. In the Astral Plane you also don't have to worry about the passage of time, so you can stay together forever even if she doesn't become a vampire herself. Ascended Astarion wouldn't mind the adventure either, seeing how he's open to test his new powers in Avernus if he goes with Karlach - I think the logic here would be the same.
Shadowheart and Gale can have a nice dynamic too. You can ascend to godhood with him and stick it to Shar or play as Gale and settle down in a cosy cottage, having an idyllic family life as a contrast to his previously reclusive existence. From the origin companions, Shadowheart seems to be the best candidate when it comes to fulfilling Gale mother's wish of becoming a grandma too.