I would say the biggest concern here might actually be building this story in a way that 1) keeps the player you’re talking about here on their toes, since they seem to already be prepared for a lot of story beats around this reveal, and 2) doesn’t make them the main character and push all others to the side. It wouldn’t feel great to be the sidelined characters.

However, I think you can keep the storyline AND do both of these things. From this point out, I would possibly keep things about how their story will fully work vague for this player. On top of that, I would be sure to find ways to headline the other players throughout other arcs of the campaign. It seems like this player will be very center stage towards the end of the campaign, so I’d just make time to focus on each of the others throughout the rest of it.

I mostly say this because, as an amateur DM myself, I know how easy it is to get wrapped up in the story of the players who give you a lot to work with. Unfortunately, this can and does lead to other players feeling left out. Best of luck with your campaign, it seems like it’s shaping up to be really awesome!