I've found that packing light for cooler and wetter climates than Tucson is just as easy as for warmer ones. Like in the past, my carry-on 2-wheeled roller bag for my next trip will either be my "big" one (22x14x9, 41 liters), my International one (20.25x14x8, 36 liters) or my new European-sized one (19.5x16x8, 38 liters). I'll keep the full weight down to about 20 pounds.
Sometimes the trick of packing light has more to do with the shape of the bag and how the packing is done than with the bag's capacity. And, of course, with planning to wear some things more than once and doing laundry along the way.
I think layers are better and more versatile than "thicker clothing." Include at least one thin wool top if you can tolerate it. Fleece if you're like me and can't.
Take something for rain, preferably with a hood. Include a watch cap for when it isn't raining or if the fleece lacks a hood. Also include a pair of gloves, and a scarf or 2.
Lightweight long underwear bottoms combine well with lightweight, easy wash and hang to dry pants for added warmth. A light long underwear top under any other top adds warmth and stretches the utility of that top.
For me, breathable waterproof boots and socks that will help keep my feet dry and warm are essential, especially since I wear them all day, everyday on my trips. Many recommend Smartwool socks. I like Sockwell medium compression ones.
So the items I listed were base layers and outer ones, but nothing in-between. The reason for that is an amusing story on the forum from years ago.
The OP was very concerned about how she pulled her travel wardrobe together and wanted everything just right. When she looked at her pictures at the end of the trip, she realized that in almost every one, she was wearing the same coat, scarf and hat. The lesson I learned from that was to pay a bit more attention to my outerwear when I travel where I'll spend a lot of time in it.
In 2016 I did a 6-week trip to England and Scotland from May 1 to mid-June. I did have and used all the types of garments listed above, but it was much colder than I expected. I didn't have a fleece top of any kind and that would've made all the difference. I think a puffer jacket would've been overkill.
I'm planning a 5-week trip to Ireland, Northern Ireland and Wales for next summer. I've been monitoring the weather for Dublin, Belfast and Cardiff. The temperatures in all 3 have been running from mid-50s lows to mid-60s highs with rain on most days.
Of course I'll check timeanddate.com for historical data, and for current weather before packing, but I'll probably be packing the same layers I listed above. There's plenty of time to work out my typical in-betweens of 8 or fewer tops and 3 bottoms, including what I wear on the plane. I'm not a "2 neutral colors and an accent one" kind of person. I tend to use a scarf or a picture for my inspiration. Like this scarf. Or this picture.