I have multiple tables like this for my vegetable garden. They serve their purpose but I’d like them to be more stable. The 5 ft vertical posts are very wobbly and that’s a big part of my concern.
I can see a couple of options, diagonal bracing is one - shown as the brown X, a joint between the two braces where they cross will also help.
But another option can be re-inforcing triangles in the corners shown in blue - these can be larger if you wish, also depends on the strength of the horizontal member.