I think this issue is one that's recurring throughout the game at various points and isn't unique to this romance. I find that pretty consistently, the game doesn't approach Tav as though thy are a character with an internal life beyond the most shallow surface level. I theorise that Larian's philosophy overall with his game is rather Tav and to a large extent Durge are both simply vessels for the player to interact with the game world. Stuff happens around and to them and they are the method of we the player reacting to them and seeing what happens as a result.
I know what you mean, though I currently feel like Astarion's romance is one of the "worst offenders" in this regard. I admit, this might me in part because I have just played with Gale as a romanced companion before. So I have the direct comparison to writing which allows me a fair amount of flexibility as well as consistency in portraying positions my character might hold, but also grants me a chance to be vulnerable - and be consoled as a consequence - and maybe most importantly: allows me to get into arguments that can actually get resolved amicably.
With Astarion many dialogue choices that would seem relevant are purely cosmetic. If you progress the romance in the act 2 scene mentioned above, you can choose between hugging him, opening your mind to him and another option which all have the same none-comital reply. You'd assume that if you open your mind, the narration would let you choose what he sees, or at least tell you, as this seems to be a relevant information moving forward. But no, it's pure fluff.
There is also the issue that the two scenes in act 2 have, to me at least, vastly different outcomes: The post-Araj scene is, as Zayir already mentioned, selfish. Astarion is moved that someone is seeing him as more than an object to be used, but he does not feel the need to return the favour. In this scene "this is nice" means "It is nice to be cared for". The post-Yurgir scene starts with Astarion's confession, here he feels bad for having wronged you and expresses his desire to figure out what a real relationship might look like. "This is nice" in this context means the none-physical relationship. Moving forward, both scenes are treated as the same, it does not make a difference which one you got.
Worse yet, while both routes continue with a dry-spell, none-sexual intimacy is never actually explored. Which is a shame really. I think it could have been extremely rewarding if how Astarion approaches the ritual, would change depending on whether he remained largely selfish (Araj-route) or tries to have "something real" with the PC (Yurgir-route).
But that was a long rant, only to say that I for the most part agree with you. XD
