Well that’s a bit different. Sorry if my initial criticism came off has hash by the the way.
I still think the premise needs a lot of work. From what you are describing it sounds like there would be a lot of dialogue between Gale and Mystra, and as a personal DM rule I try to limit dialogue between NPCs. In my experience as a player that isn’t very engaging. Listening to a DM describe two characters falling in love sounds like a lot of time listening and not role playing.
If you want to hide that the story is about Gale’s origins, you could have the campaign begin with Mystra (in disguise) tell the party that Karsus has been reborn and task them with hunting him down. Then when they find Karsus they discover that it’s actually Gale, that the person who sent them on the quest was Mystra, and that the two were lovers. At that point they go from hunting Gale to accompanying him on his journey to the astral plane.
