Puzzled by how little it took for the gears for S Korea, I checked the 4 Rivers trail and it indeed seems mostly flat, but there is one peak in the middle. ...
I cycled up that pass once again on my recently completed tour of South Korea, this time from south to north, in order to hop over from the Nakdong to the Ocheon river paths. Once again, a 11-36t cogset was all that was needed given my load.
The hillier, more climb-intensive of the several official cycling paths in the ROK is the East Coast path rather than the Four Rivers path. In the former one travels essentially from bay/cove to bay/cove, and must climb out of one to reach the other. The other factor impacting choice of gearing along the East Coast is head winds.
No breakdowns either on this second, longer tour. The only times I took out and used the multi-tool thing that I carried was to adjust the rear brakes so they would not rub against the rim after unfolding the bike. Other than that zero mechanical problems and breakdowns, or tire punctures.