Romance between Astarion and a custom Tav feels most authentic when you let their power imbalance breathe. Too many fics rush to make him soft and devoted after the confession, but that ignores his two centuries of survival tactics. He's manipulative by nature, even when he doesn't mean to be. A tip I swear by is to keep that edge in their post-game dynamic—little moments where he reflexively tries to charm or test Tav, only to catch himself and feel genuinely confused by his own honesty. That internal conflict, the unlearning, is where the real romance blooms. It's not about grand gestures, but the vulnerability in letting a guard down when every instinct screams to keep it up.
Also, please give Tav some flaws! A perfect, endlessly understanding partner turns Astarion's growth cheap. Let Tav get jealous, or frustrated by his evasion, or make a well-intentioned mistake that triggers his fear of control. The romance strengthens when they have to navigate real friction, not just bask in mutual adoration. My favorite read lately had Tav accidentally using a phrase Cazador used, and the fallout—Astarion's cold withdrawal, the painful talk—made their eventual reconciliation so much more meaningful than a smooth-sailing ever could.