If hiking from Rainy Pass to the Monument is tougher than normal for any reason there is a lower elevation route to the Canadian border. You wont hit the official Monument for the PCT but you can ‘touch’ the border.
Ross Lake also offers a water taxi to the upper banks of the lake and then you can walk to the Border on a road from Hozomeen Camp grounds.
Or you can take the following trails from Highway 20 along Ross Lake to Hozomeen Camp Grounds.
From Highway 20 take the East Bank Trail to the Lightening Creek Trail to the Hozomeen Lake Trail and from the Campgrounds you can follow the road to the border.
I am not sure how well maintained these trails may be and you should always contact the Ranger stations that service these area’s before any hike.
However, the East Bank Trail is part of the official PNT trail and should be pretty well maintained.
I have read reports that the Lightening Creek Trail needs work. Blow downs and over growth on the trail.
I have not seen information about conditions for the Hozomeen Lake Trail.
Now if you want to start at Ross Lake and make your way to the official PCT Monument there is only one viable way at this time from all reports that I have read about trail conditions in the last 2 years.
Starting with the East Bank Trail which is a part the official PNT track.
Turn onto the Devils Dome Loop Trail. From this trail you join with the PCT.
Continue on the PCT/PNT to Castle Pass.
From Castle Pass continue on the PCT to the Monument.