Recent romantic storytelling has evolved the "final relationship" into something more nuanced. We now see the "Deredere Tsundere"—a girl who has completed her arc and is now in a stable relationship, but the Tsundere habits remain as quirks , not defenses.
"I'm not saying I'm going to be nice to you all the time. You're still an idiot. But... you're my idiot. And if you even think about leaving, I'll never forgive you. Stay. Please." Are you looking to write this as a script for a manga chapter for a light novel , or perhaps a character profile for a game? Lovely Sex with Tsundere Girl -Final- -Completed-
Her final relationship is not a "fixing" but a . She remains sharp-tongued, proud, and easily flustered. But now, those traits exist within a safe container of trust. She can be tsun because she knows he won't leave. She can be dere because she knows he's earned it. You're still an idiot