I’ve implemented a working solution for now, which is to input everything as a ‘flat’ no column post, this way the ‘flow’ of the content when the client is entering is exactly how it would be on mobile.
On Desktop, I apply width:50% and masonry plugin to all direct child elements of the post-body container.
On Mobile breakpoints in CSS, I make the child elements width 100%.
It’s working well for now, although as usual, certain combinations of block heights might result in unexpected layouts.