My brother operated a lawn and garden business on Friday Harbor, San Juan Islands WA...One of the invasive plants he complained the most about being a nuisance was the blackberry...
Hard to believe a fruit bearing plant like the delicious blackberry could be a nuisance, but he often resorted to burning them out...Not sure if that can be done these days...
I too am in the Pacific Northwest like your brother was. I can attest to the blackberry zombies.
I once asked a local master gardener on his live radio show (Ed Hume) how to get rod if the blackberry bushes that keep re-emergence in my backyard. He told me there were two ways.
1. Bulldoze out all if the soil and replace with new soil. 2. Cut off the blackberry stalsk near the ground come fall and paint each one with roundup. It sucks the roundup down into the underground roots as they prep for winter.
Then he said “really, nothing works.”