
I suspect its entirely a contrivance to have the fun of this trope. That the genre has dragged it to death doesnt mean a story like this isnt kinda the original setup it was made for: two people that already know and trust each other but one person is too driven and the other is too insecure to risk the relationship by confessing.
I dont mind it. The trope got popular because, when done well, its quite delicious. On that note; hey Perez, did Tia bring her own engagement ring to the negotiations or do you just permanently keep one in your pocket just in case?

While they have feelings for each other, the law of their empire is that the empress (would be tia's position in case of marriage) cannot be a matriarch (the position tia actually wants) due to conflict of interest. So for now, whatever feelings they have with each other is not their priority, as the title says, tia will be the matriarch in this life.
Spoiler: they'll make a way for her to be both later on, let me know if you wanna know how, it is major spoiler though

Perez cannot change the law since it will be abuse of power, what he will do is he will threaten the emperor that he won't be accepting the position of the crown prince if he will not sign the change in law that would allow the empress to be the matriarch of her family.
At this point in the story, Astana is already revealed as a failure of a man and it would bring shame to the King if Astana will become the crown prince. And since he has a huge ego, he will accept Perez's proposal and sign the law change. This is all while the King is on his deathbed iirc since the empress will poison him.

i see your point, I guess we just haven't reached the part where they break down the contents of the contract. If the contract is not time-based, but instead about perez implementing a law allowing tia to be the matriarch for their marriage to come about, i'll be quite happy. That would be a refreshing take on the contract marriage trope
But didn't we already have the scene where perez confessed and tia accepted? Why are we unnecessarily adding the trope of contract relationship? For manufactured drama? To leave open the possibilities of future misunderstandings? For the fl to have 20 episodes to bury her head in the sand?
Don't get me wrong, contract marriage is one of the junk-food tropes i like, if done correctly. But here it is so pointless