I did not finish reading it so only know a bit of the plot MC transmigrates into a cannon fodder. Previous owner of the body raped mL and got himself pregnant. ML married him for the child and threw him in a mansion, not visiting ever. MC transmigrates and tries bring ML back so they can divorce. He also has some sort of space from where he finds a flower that eases sleep. He gives it to someone and makes good money???
I think it's 'Puffed-up after Giving Birth to a Wealthy Man’s Heir' https://www.novelupdates.com/series/puffed-up-after-giving-birth-to-a-wealthy-mans-heir/
It's not this one. There wasn't any switched babies scenario. And MC is a transmigrator and was already quite pregnant when crossed over. Found it! The title is — I’m Pregnant With a Wealthy Old Man’s Child