We have done this (twice). We walked off early (8am- ish) with luggage and took a pre-reserved shuttle one time (pre-COVID) and the other time we took an Uber/Lyft that we pre-requested. Both were on a weekend day and got to the airport in about 35-40 min. We have TSA pre-check and flew through. We sat around, looked at shops and realized we had more than enough time. If you are leaving on a weekday be mindful of commute traffic because that will slow your trip to Sea-Tac, but you should still make it if you are off the boat first thing.