Is there a common route that Seattle GA pilots use to circle Mt Rainier while avoiding class B airspace?
I fly out of BFI and RNT.  The short answer is go East to clear the B, then South to parallel terrain as you climb.  You will have more time to do this if there is a North flow.
There’s no specific route per se, just climb as best you can while keeping between the two.
The Lake Youngs departure gets you going in the right direction, and once just past Enumclaw you can make a circling climb to get comfortably above the ridges.