I always recommend after a bike trip, to make your exact list of what you took, marking down what was in each pannier, and your thoughts on stuff like:
-what worked well for the temperatures/weather you encountered
-what didn't work-what you never touched and really didn't need
-what you said at one point, "damn I wish I had my X or Y" (sometimes tiny useful things like nail clippers or whatever
-what thing you had but maybe a lighter, better, smaller version would be better--future purchases ideas
all this really really helps with enjoying a trip more, but clearly very individual
For instance, I bring an inflatable camp pillow--better sleep for me totally totally worth it