After seeing all the Starship failures, having something with a membrane keep things in place seems like an obvious solution. Has it been considered or tested - or do we simply lack a material that is elastic at the (cryogenic) temperatures required? What would it theoretically take? What subjects would you need to study to develop such a material?
Many propellant tank, especially those required to work in zero-g environments, do use just such a bladder-inside-a-tank for the fuel. Typically monopropellants for thrusters.
It completely removes the requirement for Ullage of the propellants, but adds complexity, cost, mass and failure modes.
Additionally, flexible bags are a bit hard to make at deep cryogenic temperatures.
Additionally, flexible bags that can contain 200 tons of cryogenic liquid sloshing around under 5 g of acceleration are.... simply impossible to manufacture.
I don't think we will ever have the technology to make a soft bag that can hold 1000tonnes (200 tonnes under 5g) of cryogenic liquid that is sloshing around.
For interest: take a peek at this. Work on developing a bladder that is cryogenic compatible. But max size very much small.