I've written a companion quest for Aldiirn through act 1! It's on the comic site: https://bg3.mrcrowb.art/the_musical_merchant

Aldiirn's act 1 romance is mostly about sharing his music and getting to peel back some of his customer service face. There's a few hints he's ace, and to reach partnered status in act 2 the player would have to be okay in a sexless relationship. I haven't thought too much past that for romances though. Without romancing him, it's a matter of approval to get him to be honest about himself.

I'll likely work on fleshing out Aldiirn's companion quest as I write the comics since I want to touch on some of it there too. His quests kicks off with finding missing couriers from his clan and the outcome is tied to their fate. Aldiirn's Good end has him join Iaurrhen's leadership and make headway with uniting the Underdark via cooperation. His Evil end would also have him in Iaurrhen leadership, but in a more micromanaging dictator role where everyone in the Underdark must follow his vision or keep wallowing in chaos. His Bad end sees him disillusioned with the idea of a united Underdark and he joins the Zhentarim or something.

Good or Evil ending, a player would have to follow him to the Underdark to maintain the romance, he can't be convinced to stay on the surface. Bad ending, he'll do whatever the player wants, but will be unhappy on the surface.