Yes, I think you would either have to take the train to Montreal, then connect through Vancouver. Or, fly Quebec City to Toronto and then a direct flight to Seattle. While Toronto Pearson is a nightmare at times, the upside is you will clear US customs in Toronto through a dedicated in-transit passenger line, and remain "airside" at all times. Your luggage will be checked all the way through to Seattle.
Now - Porter (mentioned above) does fly to Midway in Chicago, so that could be an option if you can get flights from Midway to Seattle (I assume so, but never tried, so I'm not sure). But you would have to connect at the Toronto Island airport - so two connections - and would clear US customs in Chicago. I haven't done that post-Covid, but it is a fairly straightforward process as there are few international flights into Midway.