When are you sailing? If after July 4, I will post our experience with the cross country rental. Right now (and this can all change) we are planning on driving immediately to the border crossing, stopping at Fairhaven (Bellingham), Chuckanut Hwy W-11 (with possible stops), stopping in La Connor, back to I-5, Future of Flight (if time and stamina allow), and into Seattle by dinnertime.
You can check border crossing waits on the internet. There is also an app for this (Canada Border Wait times)! I plan on checking it when we head out. If the Peace Arch looks good we'll cross there. If not we'll go the extra 15 minutes to the Aldergrove border crossing (we're traveling on a holiday so it may be busy). Another great resource is historical border delays:
https://www.th.gov.bc.ca/ATIS/historical.aspx. You can see what historical border crossing wait times have been for the timeframe you are traveling. Lastly, since you're also coming from Seattle to Vancouver, there is an app called Canada eDeclaration that claims it will help speed up your border crossing by completing your declaration before arrival and getting a QR code.
Good luck with your planning!!