Currently this isn't possible - when the columns collapse on mobile the left will be above the content column, and the right column will be below those. This is true in all the themes. So in general using a right column rather than a left column might be more appropriate for you (which I think is what you're already doing!).

Have a look at the Column Stacking info on here.

It could be possible to add a mode where the stacking order is changed. e.g., maybe it could go in order of Top, Content, Left, Right, Bottom in on mobile screens.

I think this would need to be optional rather than standard because some sites might rely on the left content coming first.

Do you think this would be valuable to you (and does anyone else here think it would be useful?) or is using a right column good enough?

Thanks

Joe