Review with spoilers up to chapter 6. It's trash up to now.
Weak minded MC, he isekaied into the body of a weak ugly and fat guy, but so what? The fat guy owner of the body has a stronger mind than the MC, it's crazy! I will call the owner of the body by "Butao" and the Isekaied guy into his body MC from now on.
MC failed to protect his little sister, Butao intervened and protected her.
Butao even being fat aspired for a beautiful girl. Well she is a bitch that we all want him to break up engagement with but he still had a good vision for looks at least, while MC didn't even think of using it.
Honestly, in MC's place I'd use the fact that she wants to break marriage, to write a new contract stating that he could use her as a "helper (slave)" until the end of academy to dive/explore the dungeons, of course ONLY for dungeon exploration purpose, nothing more, it's a contract/deal that she has to accept in order to break the marriage contract after the academy ends.
With that, she being strong as she is, I'd have the MC use her until the end helping him find all the best hidden pieces, items, etc, and monopolize it for himself, and protect the ones he need to protect, such as his little sister, family, and possible new characters that appears who aren't that bitch he was engaged with.
And of course, after using her until the end, after breaking engagement, would tell her: "I hope we never meet again. Goodbye."