Regarding the sleeping bag, mine always went in the pannier when I used panniers. It goes in the same place these days except dry bags have replaced panniers.

For stuff on top of the rack I have always preferred lightweight straps.

Tent gets rolled with the bottom on the outside. No need for any further water protection. Ground sheet, I don't use one. Sleeping pad goes in pannier, but when I used one that didn't pack small it went on top of the rear rack.

Various folks that I have traveled with carried the pad in a stuff sack big enough to stuff the sleeping bag inside with the pad around it. Seemed to work well enough.