Try modest incremental CCW turns, say 3/4s, on each side. Then close the hatch, push the key button, and observe if the hatch springs up slightly on latch release indicating snug closure.
If it does, road test that adjustment.
If there's still more rattling over bumps/expansion joints/tar strips than you prefer, use the 6mm hex wrench to make a further 1/2-3/4 CCW turn. Etc.
The snubber mushroom cap is easily removed and reinstalled but you shouldn't need to do that.
The adjuster "screw" held in the bumper base has only ~5 threads which is enough. Too many CCW turns of it would release it from the base.
If the bumper cap is pulled off, you must turn the adjuster screw CW until it snugs lightly before trying to push the cap back on to the base. Only then will the bumper cap easily fit onto the base riveted to the hatch underside.
Lijewski: "Sum Ergo Drive-O. Mucho!