I think the protagonist was reborn in a game or novel he was playing. He lives with his father and mother on a floating island. Polygamy isn't an issue in that world, however the women have the say here and men are often treated pretty bad. His mother is kind, but the second wife of his father is a higher noble who treats him and his brothers as chess pieces, and sells them to older, ugly, somewhat rich ladys.
The protagonist doesn't want his life to be like that so he leaves the island on a flying boat to achieve riches and weapons etc. To escape his stepmother claws. With his memories from his pas, He finds an island with old technology. Later with his achievements it's possible to attend a school for which his stepmother wasn't willing to pay for.

Otomege Sekai wa Mob ni Kibishii Sekai desu

~4400 manga completed
~1700 ongoing...