There's no obligation to torque the stem bolts to 5 Nm. Torque wrenches can malfunction and cause more problems. Use a short hex wrench and moderate force.
Pictures posted show that expanding plugs are supposed to support the steering tube against clamping force, in addition to allowing the headset bearings to be adjusted and not slip.
I avoid these problems by filling the entire length of the steerer with epoxy, to at least the bottom of the stem. You could go further to the upper headset bearing. A thread insert is placed at the lowest point, so the steering tube can be cut shorter, if needed. Most often, my thread insert is a star nut, ground to a smaller diameter, so it slips easily into the steerer. A long M6 bolt is covered with a plastic drinking straw or heat shrink tubing and screwed into the star nut, to prevent adhesion of the epoxy. Grease the star nut threads too. The steerer is plugged with foam rubber or core foam, just below the star nut. I try to use stem angle to avoid using more than 20mm of spacer above the headset top cover. Two of my bikes have no additional spacer, with -6 stems. On the third, there's a -17 stem and 15mm of steerer that could be cut off.