Your prep looks great, Rod! What kind of rub is that?
Since you're new to ribs in your SI, I would start checking them at 4.5 hours. Use a toothpick - tender in, and pull meat sideways from the bone, all good! If not, go another 30 min and try again. 5 hours is not a hard and fast rule - it all depends on the ribs! Those look pretty meaty, so I would suspect you're looking at 5+ hours, but you never know. Don't forget the water pan next to the smoke box!