I feel the same. All of the companion storylines seemed to end short, but since I played romancing Astarion, here's my two cents:
1. The build-up in the Act 1 is BRILLIANT! I loved it! There were enough moments when you get to talk with him about what's going on and what he thinks. When the building blocks for this companionship were laid out, I thought that this is going to be marvelous - maybe the best romantic storyline I've got to play so far. It was difficult and hinted that some hard decisions might have to be made. His own storyline in the Act 3 is really well made and heartbreaking. Up untill the moment when I had to decide which path to take with him,
(whether to encourage him to ascend or not)
I was determined to make the only evil decision just to make him happy, and when I learned what was actually at stake,
(killing 7000 spawns instead of just Cazador and the siblings)
I thought it was actually great writing. Suddenly it felt a genuinely difficult decision. Can I make such a horrible decision for him? Should he? I tried both decisions and was impressed how deeply meaningful both of them felt. Bravo, I thought. My heart was racing and I was so, so invested in this.
2. And after that...? After that you are given a new chapter in Astarion's life. Either of the decisions leaves a mark in him and he has to go through something very serious and it takes time and it changes him and how he is in the relationship. It's realistic and feels important. I loved it. However, I felt that romanticly the other path was suddenly a deadend.
(If he ascends it's either his way or the highway)
It felt a bit... weird, maybe? That as a player I didn't deserve this. That I should have been able to affect him on this matter. But, in the end, I accepted that alright, maybe this is such an all-consuming turn in his life that this is how the story goes now. Personally I ended up continueing my playthrough with the other decision since I felt it made more good for him and the relationship. I actually even liked the idea that this would leave a horrible hindrance looming in the future.
(Him being affected by the sun once the tadpole was erased.)
Both of the storylines with him seemed to have something good and something sad and I loved it. I was really curious to see what comes next. How does things evolve now that he starts to go through all this and to reflect all that just happened?
3. And... nothing. During the rest of the game I kept anticipating this storyline to continue. I mean, it had just started a new chapter, and it was so really, really, oh-my-gosh really interesting! What's going to happen next? I kept playing the other storylines patiently waiting for something to trigger Astarion's storyline to continue. But it never did. I kept thinking that oh, that's weird, maybe I have to finish this quest or that quest, or maybe I have to have some long rests, or talk to him during the quests, and eventually I started to think that there must be a bug. The idea that the story just wasn't going to continue didn't seem even possible. I played dozens of hours more just wondering how long it takes before this continues.
4. And then comes the ending. I anticipated that we'll have a final gathering with the companions and I'd get to experience how their stories deepen before the final fight. I was wrong. So I ventured forth to fight the last battle thinking that at least then there must be a wonderful epilogue where all of these unfinished storylines would finally get to flourish. Enter the dock scene.
First I saw a cut scene telling that Astarion can stay in the sun. I was thinking that oh, that's unexpected and sudden, actually even a bit bummer that it resolves all on it's own, but on the other hand, I'm happy for him now, so great. Then I have another cut scene about someone else. Then a cut scene where Astarion starts to burn in the sun afterall, and he has to run to safety and someone makes a cheap joke on him. I felt... shocked. Like... what? WHAT!? Why was I first told that he is okay and then immediately told that he is not and why is it told like this and why is he laughed at and why can't I go after him!? Up untill that point I felt that all of the characters were treated with respect, but suddenly Astarion is just rediculed. Why? Instead they could have wrote that he starts to feel the sun and he has to go for now and have a quick conversation on how we are going to see eachother in a moment. Something emotional, like the scenes before had been.
After the dock I still expected some kind of epilogue (yeah, maybe I have a thick skull). Instead I got a cut scene with Astarion, where I could pick something like two lines of conversation. The whole conversation felt really off. Distant. Light-hearted. Nothing of a reflection of the previous storyline. Where was that redemption story, where the emotion and connection? It just wasn't there and then the credits started to roll. I felt so... empty. And baffled. I had to google to verify that I probably hadn't had any bugs, but that this is how it actually goes.
So when all is said and done I'm torn. Astarion's storyline started as one of the best romance stories I've ever seen in a game. I love it. It felt deeply meaningful and was the highlight of my playthrough. Untill it wasn't. During most of the Act 3 this storyline just didn't exist anymore and the ending was blunt. I hate how this turned out and can only wish some kind of re-writing.
