Yeah so this is a classic example of competing priorities. You want a newer home, with land, and an easy commute to the Seattle Center, without spending millions. That's going to be a tough one. Commute-wise I think you're definitely on the right track having a bias toward transit. Traffic is already terrible for drivers in the general downtown area, and it won't get better as the city is gradually reprioritizing street lanes to more space-efficient uses (buses, bikes).
How locked in are you to the Seattle Center commute? Do you expect to be there for many years, or is it reasonably likely that you would eventually switch to something closer to the actual central business district? This matters a lot for commute planning! Light rail doesn't get you too close to the Seattle Center, but it's definitely the way to go for a lot of the other downtown office jobs. Maybe the Link->Monorail transfer at Westlake would make sense for you? Not sure, I've never tried it to see how quick and reliable it is. For a direct bus ride to the Seattle Center you'll want to look in Ballard, Phinney Ridge, Greenwood, Queen Anne, parts of Magnolia. The D Line from Ballard/North Beach and a couple of Magnolia routes go along the west side of the Seattle Center, while the 28/5/E Line go down Aurora and therefore get you pretty close on the east. The problem with all of these neighborhoods, given your criteria is that most of the single-family homes are from when these neighborhoods were originally developed a century ago. If you want newer construction in these neighborhoods that isn't a townhome you're probably starting at around the $1.5 million figure you find "absurd" (even though you could probably maintain a reasonably high savings rate even spending that much). You might have better luck looking for older homes that have been more extensively renovated in recent decades. New kitchen, modern wiring, these types of things. Those can be had for less.
One perhaps creative idea would be to buy an existing home with a big back yard, put a DADU in a corner of that yard, live in the DADU, and sell the existing house separately. People have been forming two-unit condo associations to enable separate ownership in this manner. If you're the one forming the condo association you could certainly write into the documents that the entire back yard is yours, and the other unit has control over the front. How big of a house and yard do you require, exactly? DADUs in Seattle are allowed to go up to 1,000 square feet, plus garage and basement space doesn't count toward this limit.
Alternatively if the Link to bus/monorail transfer might work for you, you might consider going to the Eastside near one of the light rail stations that is set to open in less than two years. The houses there tend to be newer and sit on larger lots than you'll find in Northwest Seattle. Trade-off being, of course, that the neighborhoods are less walkable.
