What a great thread.
I've done a fair bit of travelling in the past few years. Aside from my frequent shorter trips to Toronto, New York, and other spots, I spent a week in London last February. It was freezing the whole time; I swear it was colder there than in Canada at the time...layers were key. I also spent several weeks in China plus a week in Vancouver with my family last fall. I composed threads about this -- filled with great advice like you've received.
Because on those trips I was doing a wide variety of activities (from hiking to dinners and theatre outings) I found I needed a variety of clothes. Gaylene's advice to create and try on outfits for each type of occasion was really helpful. I did as she suggested and tried to imagine a "cooler weather" version of the outfit, a "rainy weather" version, a "heat wave" version. I wanted to look good whatever the conditions.
I didn't manage to travel with a carry on, partly (I think) because of the time of year I was travelling (or the varied climates I'd be experiencing within one trip) and partly because my carry on bag is old and not a great design -- a new bag is on my list of items to purchase. Truthfully, I could have managed the carry on for the London trip, except I wanted room in case I made purchases (which I didn't end up making in the end.) And Mr. Suz was checking his bag anyway, so I went ahead and put my stuff in the larger bag.
For me, it's most important to consider footwear. I know they say to bring one or at most two pairs of shoes, but unless my trip is short and my activities more focused than is typical for me, that simply doesn't work well in my case. I like to have different shoes to go with different kinds of outfits, but more than that, I find my feet stay happiest if I switch up the footwear frequently. And shoes take up room.
So do sweaters, and jackets and coats -- which is why travelling in cooler temperatures makes using a carry on even more difficult.
May is transitional weather, so you'll need clothes for cool temps and clothes for warmer temps. But smartly chosen layers should go a long way.
It sounds like a fantastic trip, and I am taking notes from the lists -- I hope anyone who has one will make it public either here or in a separate thread.