The sunlight is the one thing he really enjoys from the start of the game on. That one moment of genuine bliss, when he is trancing in the early sunlight, is personally what "saves" that incredibly fake night together with him for me. And it's more than just light and colour, being able to walk in the sun is being part of the world instead of having to hide in the shadows. If you look at the dock scene, the group is in the process of finding a tavern in which to celebrate their victory together, when the sunlight casts Astarion out of the group. Like the heaven telling him: You don't belong. That is harsh.
They also never treat the subject lightly in any other situation. It is a topic that is part of his dialogue during the epilogue party when you are friends, you can talk about finding a way to cure him with Minsc when in a relationship with Astarion and even in my otherwise very sweet romance dialogue for Origin-Astarion and Gale, they snuck in a line about how it was difficult for the two of them to settle into their living arrangements at first.
And yes, I agree portraying them as friends by the end could have been handled better, though it works in some cases - and also it depends heavily on banter playing and such.
