As your maps and graphs show, Bangladesh is a marginally habitable area of swamps and mud-flats subject to flooding every Monsoon. It has always been this way, but overpopulation forces people who can’t afford the higher price of higher land live in the swamps and flood plains. That is why the flood areas are populated by the poor and illiterate: nobody else wants it.
You don’t need Climate Change to give people a reason to want out, they are already under stress and would leave if they could afford to do so, and if anyone would accept them. When they do cross borders they will be economic refugees, not Climate Change ones. What Bangladesh needs is a stronger economy, so they can afford to build levees and sea-walls, and dredge rivers to reduce flooding, while building up the surrounding land. They don’t need to be used as pawns in Climate Alarmist propaganda, they need to be traded with so they can earn the wealth required to address their present and future problems.