Wood is thicker, denser, and has more structure than paper. My guess is that paper is a closer analogy in this case.

This is probably accurate for films and fabrics, but I wouldn't be surprised if thick solids would be at least as slow to rot as wood. I don't know what proportion of plastic mass is in each form though.

Wood is a nice analogy because when it was first evolved there was nothing on earth that could break down the lignin. As a result, we had a couple thousand years where wood just piled up like a global waste crisis. Eventually fungi got really good at dealing with it

We're again seeing a waste crisis due to a novel material and once again fungi might end up saving us (though we don't have a thousand years to wait this time)

It was several million years

Paper is porous. Metal sheets are closer to it.

Any biological digestion of plastic will be either a very slow process focused on microplastics or have a large mechanical component.