For 3-4 day PNW backpacking (wet, variable loads), skip the 50L-it’s too cramped for a tent (esp. a 2P like Big Agnes Copper Spur), compressed sleeping bag, stove setup, layers, and rain gear. You’ll be jamming stuff in awkwardly, and wet items bulk up fast. Go with the 71L Medium; it swallows all that plus food/reserve space comfortably (I’ve fit 35-40lbs gear in mine for similar Olympic NP loops).
Backpack carry? Yeah, stowable harness works for short hauls (airport to trailhead or camp shuffling), but it’s no internal-frame pack-straps dig on long miles with >25lbs. Use it as duffel primary, backpack secondary. DWR coating sheds rain well, but line with pack liners for submersion. Paired mine with a REI Flash 55 for the actual hike. Solid choice overall.