Lifelong Northwesterner here. Last 34 years in Willamette Valley--home of Eugene/Salem/Portland.
To your original question------most of the cloudy grey days are in the winter early spring. Late spring, summer, and early fall have lots of good weather without rain/overcast.
As you stated, if you take a couple months travel, say Jan/Feb, or Feb/March, you can eliminate a large chunk of the really rainy/overcast time here. Generally, climate is moderate. Not many days much below freezing most winters, maybe down in the 20's. Snows enough to stick maybe 3 or 4 days a year, and soon melts.
And some of the "best" days in my book, are when it is cold and clear in the winter. Mt Hood, Mt Jefferson, Mt Adams, Mt St Helens, the Three Sisters intriguing your views.
No sales tax in Oregon. The price label you see is the price you pay. There is state income tax, but SS exempted.
Try it. You'll like it.