I would second a prior recommendation for Pacific Power. Both Coastal and Pacific Power (the two Volvo dealers) seem equally (that is, highly) regarded, but Coastal quoted me $2500 for an annual service and Pacific Power just did the same work under $1700. Both will come to wherever your boat is in the greater Seattle area, just charge you for the travel (much more reasonable than it sounds).
For an actual haul out and general work, CSR seems to be very widely considered the best yard in town. I’ve been working with Nigel (one of the service managers there) on and off for ten years and he is helpful, honest, and reasonable (referring me elsewhere when it’s a job like refrigeration that CSR tends not to do). Miller and Miller is good for refrigeration and systems (and they have a small haul out / yard operation); Pacific Fiberglass is where I would go for any damage or hull repair.
CSR absolutely allows outside contractors into their yard. Your engine can be worked on by Pacific Power or Coastal while hauled at CSR. I believe they also allow a wide range of outside contractors.
Canal boatyard is also very well regarded, but it’s more of a DIY yard where you pay them to haul and block and others to fix your boat or DIY (though I think they do have some services on site).
There are a few other good marine services companies around (MonkeyFist at EBM is a good example), but for yards I wouldn’t take my boat to anywhere else than the three mentioned above. There are some really shady yards inside the locks and lots of horror stories if you talk to other boaters...