Yeah, a 9 meter diameter one, which adds mass and volume and complexity and detracts from the payload.

Instead what they do is use thrust to accelerate the whole vehicle a little, which presses all the liquid into one end of its tank where it can be pumped out. Instead of carrying special settling thrusters, they originally planned to use ullage gas for this but it's not clear that can work.

deeper discussion with math: https://forum.nasaspaceflight.com/index.php?topic=60124.60

plastic balloon?

pretty much everything, including and especially plastic, becomes a fuel when it comes into contact with liquid oxygen. With liquid oxygen in contact with a fuel you're virtually guaranteed a fire at some point as it takes very little heat to start the combustion. This is why when rockets tip over it's an explosion and not just a broken airframe with fuel/oxidizer leaking out.

Yes and they would be called bladders, but then you need to carry a gas to compress the bladder.

Most plastics are very brittle at the cryogenic temperatures. Also if you are using that method for a liquid oxygen tank, you need to make sure that the plastic you choose doesn't spontaneously combust on contact with LOX.

What plastic is elastic at those temperatures? (-182 °C)

Something much like this is used for wells - both simple and effective. I wonder why it wouldn't work here (or if just hasn't been tried).

Cryogenic temperatures make most materials more brittle, hard to get a material that works at a wide enough range of temperatures to make a balloon to work correctly.

If you go for a narrower range of temperatures (ie. not structurally stable above 0C), it would need to be manufactured, transported, stored, tested and installed at seriously low temps which probably negates the possible advantage with the added technical complexity.