The most beautiful old church in Seattle is Immaculate Conception. It’s a bit of a hike from Kent (probably 30 to 40 minute drive). The Sunday liturgies aren’t super orthodox. The 9:00 mass is your typical OCP type stuff, and the 11:00 mass is gospel music, which is OK by me but the liturgy is a bit fast and loose.
Anyway, since you specifically mentioned art and architecture, it really is an absolutely stunning church with beautiful murals, statues, grotto of Lourdes and intact high altar and altar rails. If you will be in town Sat eve, the vigil mass is a bit more subdued with just a piano and a cantor.
immacconceptsea.org/
BTW, I noticed that the website says “organ” for the Sat vigil - it was a piano and cantor on the occasions I have attended.