40 rpm indeed a point of great struggle. It may feel significantly easier if you can spin at least 60 rpm which likely means a gearing change.
Another option is just buy a gravel bike with super low gearing, 46 to 30t chainring and 42-11t cassette. Gravel bikes also have endurance geometry like your Giant Defy.
I don't really pedal on a 5% gradient descent or more. If you have tight fitting clothes, just tuck it in and it will get you to 60 kph.