For pooping - I'd wait 30 minutes at least. It doesn't go through a big dog's system that fast....
And actually with your bigger goldens (again on a quality food that advertises fewer and smaller poops), you should have a predictable schedule when he has to go outside for poop.
With my Jacks, it is in the morning when he gets up. And then in the evening after we come back from our walks. He might sometimes have to go in the afternoon, but in general we know for sure there are two times a day when he reliably has to go.
As you go back to square one with the potty training, I don't think you need to be as frequently in and out as you would with a little puppy. But make sure you are getting that morning outing, the outing within an hour after breakfast, the early/midafternoon outing, the evening outing, and the before bedtime outing.
Figure out what your dog's pooping schedule is (when he reliably has to poop), and make sure he gets to poop outside at those times.
ETA - And make sure you and your family are patient with the dog. 8-9 hours is a very long time to ask a puppy/young dog to hold. And like you said, he's likely pretty confused. I don't think it's impossible to "fix" if you are consistent and find a way to get him on an outing schedule. Even if it means asking a neighbor or friend to stop at the house and let him outside after four hours.