My only extended 'tour' was ~1000mi and 40+ years ago. I carried a canvas duffle bag containing two changes of clothes (no other gear) that was stuffed into a plastic garbage bag, and bungied to the top of my Pletscher rear rack. Probably less than 10-pounds in the duffle at any time, and I only weighed 135-140 back then. I never had a problem with stability of either the bike or the load on the rack.

Later, around 1980, I got a pair of Cannondale Overland panniers and used those attached to that same cheap Pletscher rack, and sometimes used an additional trunk bag on top of the rack.... Only went on under 100-mile weekend excursions at that time of my life... Again, never had any stability issues.

BTW, I still have the same bike, and it still wears the same rack!