Uh guys? That's not a Tsundere you know? That's called a Sadist. The MC even blatantly points it out on the first chapter. Those are 2 separate things.
A Tsundere is a sad fiction that is only possible in manga. Since it falls apart if the other person doesn't play along. Of course this IS a manga but that's still not what she is. The MC not playing along is proof enough of that.
A Sadist is someone who really likes being in a clearly dominant position in a relationship. Which she has very clearly expressed to our dear bargain bin MC. She made it perfectly clear she would still marry his sorry ass if he would just be her cute little bitch. Honesty him being a sad excuse of a typical Japanese MC only made him More attractive to a woman who likes lording over others.
Sadly this just wasn't MC-kuns cup of tea. A little boy who's balls haven't even dropped yet isn't ready for a woman ready and willing to rock his world into space dust.