Mounting mess up - need advice
To fill the hole, glue in a 5/8 wooden rod/dowel  using white glue, panel adhesive, epoxy, or polyurethane wood glue. Then drill into the end of that and use some sort of long wood screw to attach the bracket.

Alternatively, get a threaded insert or connector nut that accepts the machine screw part of the toggle bolt and fix that into the oversize hole using wooden wedges and epoxy putty or builder's filler.