You might be able to (hypothetically) do it what might aptly be called Seattle's sister city, Vancouver, BC. You do still need to somewhat deliberately find a spot nearish the train, or a major bus route, or just bike, but it seems like it'd be more doable here. Haven't owned a car in years.
Last time I was down in Seattle though, I noticed they were building a massive elevated (40 mile?) train thing quite far north, which looks somewhat impressive if it wraps up in the near future.
I'm not from Seattle, but it sounds like you're talking about the 3 Line https://en.wikipedia.org/wiki/3_Line_(Sound_Transit) which will join the current 1 Line and head north up to Everett, WA. Estimated completion date of either 2037 or 2041 based on funding.
When I visit Seattle I only use public transit or walk to get around. I use the light rail as much as possible, but it only gets you kind of the way to anywhere. Plan on an up to quarter mile walk to a bus stop and then probably an additional bus to actual get to where you want to go. The previous poster is right in that you need to add at least an hour to your transit time to account for waiting for connections.
Also, a large portion of 1 Line's southern section is at-grade with auto traffic.
Last time I was down in Seattle though, I noticed they were building a massive elevated (40 mile?) train thing quite far north, which looks somewhat impressive if it wraps up in the near future.