[confused sounds] i'm gonna assume the first move means romance and the adventure is aeventi being travel partners hjfhjf ask me again if you meant something else

i'd like to think that Aether would be the one to make the first move because Venti seems like a very "love them, help them, guide them and let them go" kind of person to me. Aether has to let Venti know that whatever happens between them is Aether's own choice, and he decides to love Venti. I like some classic "Aether protects Venti during the ambush/the adventure" with Venti being surprised about it (just look at his expression when Aether protected him from the fatui in the quest!). Worried Aether + Venti being quiet and solemn because he hears prayers for protection all the time, yet no one ever really tried to protect him the way Aether did (salute to Luna's healing Venti's injuries asmr)

But then again- we have Venti making the first move in canon. The first Windblume festival!! The poem!! It was basically a confession, don't you think? He's not being very subtle but I love how he's so open about his fondness and still leaves the ball in Aether's court, it's up to Aether to decide what to do next (if he wants to), he's dear to Venti no matter what :>
If we're speaking adventure specifically, I think Venti's way of doing "the first move" is joining Aether in his travels hfhjf