Yes, I live in PNW too with lots of acidic drizzle and poor draining clay. Biochar is great for us. When to stop burning it is key.
This is a really important point. I made a biochar stove based on this video:
However, they didn't explain when to douse the fire so that only pre=biochar would remain, and it wouldn't keep going until it made ashes. You will always have a little bit of ash and that's good for our acidic soils to make them a bit more alkaline. But you want max crunchy pre=biochar and minimum ash. I think the best rule of thumb is when the flames have died down to when they are barely above the burned wood, douse it. In a 55 gallon drum, it's usually about an hour/ 1 1/2 hours. When I do this, I keep getting light, metallic sounding crunchy pre=biochar 99% free of wood. The tiny bits of wood I just save for the next session of biochar. Tiny bits of ash.
The main other thing to improve on that video is just to draw a circle on the top of the barrel for the chimney, and use that as the basis for angle grinding the hole up into the chimney.