The most traumatising romance for me was with Alistair in DAO when he stayed as a warden, you didn't let him sleep with Morrigan and you died as a hero by leaving him at the gates. He doesn't even show up on your funeral - probably too grieving to handle it. That gaming moment destroyed my old, idealistic self and made me realise that sometimes doing a shitty, questionable thing will actually work out better for you. That the world is not black and white but full of shades of grey, and that heroic sacrifice is a dumb thing to do when you can live a happy, adventurous life with your lover, friends and pets instead. This is also why I will never go for Tav becoming a MF ending.

I heard that romance with Astarion can be heart-wrenching and I saw his reaction after the breakup when you refused to be his spawn and was astounded, wondering what had happened to him after watching his playful scenes from EA.
Then when I actually played BG3 and understood him better I decided to ignore my meta knowledge and fully immerse myself in the world and go for the ritual, because it suited his and my character's story. I allowed all companions to do what they wanted in their personal quests, because I didn't think my chara should interfere. These were really drastic and life-altering choices and it felt strange telling them what to do when they've all known each other for several weeks only. Everyone should be responsible for themselves and live with consequences of their actions - I didn't want to have that burden and be responsible/blamed if something went wrong. I realised that if I didn't like Astarion's behaviour after his ascension I would have to breakup with him as a result, but it was still better knowing that he'd have a great life rather than be reliant on someone else. Fortunately, I didn't have to do it.

When I played, I found myself agreeing with him almost all the time - he has more than 200 years of life experience and has seen the worst things. He knows what he's talking about, and he's right about the power and the stronger eating the weak. Characters like Wyll and Karlach seemed like naive idiots who haven't figured out the rules of the game yet in comparison to him.

During the ritual or not choice I was panicking a bit that he might lose his soul. I wanted to help him ascend but then I chose to explore all dialogue options before committing to my designated choice to see what would happen. I was disappointed with Tav's options - you convince him by guilt tripping him mentioning the other spawn, or tell him YOU WANT him to be proud of his life. It felt weird to suggest things like that to someone who'd had no autonomy over themselves and who no one'd cared about. It's basicly telling him 'you've suffered so much, but it's a hero thing to be altruistic even if it means you will be miserable going forward' or 'it's what you want but this is what I want, doesn't matter you will be miserable but at least you can feel good about yourself for being such a good guy'. The most nonsensical comment is about the power trapping him, because they don't know all the details about the ritual and how it would affect him mentally, Tav is basically throwing bs in his face considering the ritual had been consistently presented as something that would actually free him from his physical ailments and weaknesses.

And then when I used the persuasion choice for the first time in someone's personal quest I was even more disappointed how quickly he agreed with me. The whole game he wanted more power but then one dialogue option happens and he just goes along with my suggestion. He was distressed, broken, hollow, and it felt horrible seeing him in that state. Then I read that journal phrase about his fate being sealed and it all felt wrong and bitter. Then I ran into the Gur and laughed at their pitiful mercy that I was sure they'd forget about right after Ulma's death or even before that. Astarion would still be hunted by other monster hunters, maybe other vampires, and possibly many other entities. I reloaded afterwards and went through with the ritual and it felt amazing, euphoric and cathartic. Like all this pain and misery actually had a purpose. That it was a stepping stone that allowed him to finally live a proper life that'd he always desired.

Right after the ritual I told him 'You did it! I can't believe you did it!' and I loved his response 'I did... We did.' and that's how you see his sudden confidence both in himself and in the relationship. There is always 'us' from that moment on, no longer 'I don't know what you are, etc.'. He knows what he wants and how much you mean to him. He makes all sorts of plans for the future and wants you to share it with him. In his eyes, you're finally worthy of his full trust and devotion. Unlike before where he even admits you've only earned 'a little trust'.

When he ascends he's living the life he always wanted to have, which is the immortal decadent aristocrat/magistrate with power. Who is Tav to deny him his dream life? To me it works beautifully that you give him everything and then he gives you everything in turn and you become a strong, harmonious couple with shared goals.
Ascended Astarion is a winner - someone who outsmarted his nemesis and won and got everything he wanted, someone who knows how the game works and how to play it. Spawn Astarion in comparison is someone who abandons his own needs for somebody else's morals and wants and from real life examples I know things like this don't end very well.

To me, the worst romanced epilogue with him is going to the Underdark with the other spawn - you live in bad conditions in some ruin, surrounded by Astarion's past flings, you're vulnerable because everyone must be craving your blood, and you're always scared some monster hunters might find you. And then you'll eventually die of disease or old age, Astarion will be grieving and maybe kill himself or be killed by the other spawn taking advantage of his moment of weakness. No thanks to that.