Honestly, the most believable 'contract lover' trust arcs start with a breach. They've got this ironclad agreement, right? All the rules are laid out. So the first flicker of real trust isn't them following the contract—it's one of them knowingly breaking a clause, maybe something small, and the other party choosing not to enforce the penalty. Like, she gets sick and he cancels a big business meeting to stay in, violating the 'no personal entanglement' rule. He's waiting for her to cite the clause, but she just... doesn't. That silent, mutual agreement to ignore the paperwork is the foundation.
From there, it's the little domestic spy stuff. He notices how she takes her coffee when she thinks he isn't looking. She memorizes which brand of headache pills actually works for him. They start collecting these secret, useless intel points about each other, and that curated knowledge becomes a kind of intimacy. The final stage is usually a crisis where the contract's terms would logically dictate one action, but their accumulated secret knowledge of each other pushes them to do the exact opposite. The contract gets burned metaphorically, or sometimes literally, because the trust has become a better, more binding document.