The biggest mistake I see with steadiness training: handlers reward the sit, not the hold.
Your dog has to learn that staying through the retrieve is what earns the mark, not just the initial sit.
This session: send him on a retrieve, plant your feet, wait three full seconds after the bumper lands before the release. Do it 10 times. If he breaks, pick up the bumper and reset. No correction, no drama. That pause is where steadiness lives.