Hi there,
Yes, the Layout Element is the only way of applying the fullwidth page container to the entire site. Or the ‘hack’ is to use 2000px in the Customizer > Container Width.
Not sure how % would work without implementing min/max widths and breakpoints which would get could get awfully complex. Could also lead to layout issues when components rely on a set width container.
EMs are great for responsive designs but we would need to add EM values across all sizing options to really take advantage of them.