"Obviously there's a bit of overkill in terms of clothes for summer. Where I don't expect any rain for shorter trips I can ditch the rain jacket/fleece/r3 bottoms."
Short answer:
Bring a shell and a reasonable amount of insulation that'll work in wet/damp weather.
Long answer:
I've contemplated swapping out my shell for a poncho for when dry weather is in the forecast, then last week I was blasted in the face with gusting winds and a mixture of ice and rain and have given that idea an official "%^@& that!" I've been on more than a few trips where I never put on my shell but imo it's mandatory equipment for the PNW.
Insulation is personal. On this same trip, for the first 1.5 days, I stayed wet and temperatures remained between 40* and 45*. I was fine with a thin nylon t-shirt, light button down synth shirt, nylon pants, sock liners, shell, and running shoes. I brought silk weight base layer top/bottoms but only wore them to bed so I had something dry to sleep in. My hands throbbed for a while but my core never came anywhere near hypothermic. I had the option to put on my possum down gloves but decided against it and was fine just pulling my hands back into the sleeves of my shell.
I think the "don't pack your fears" sentiment is awfully cavalier sometimes, especially from people who live in warmer/drier climates. Don't go too crazy but err on the side of caution with your wet weather gear and insulation. Even when I was carrying more insulation than I do now, and a more robust pack, my base weight was still in the 12lb range so you don't have to break your back doing this (save that pain for your camera gear).
Over time you'll figure out what works for you as you experience the PNW in all its glory and will be able to make informed decisions on what to leave home.