That pressure washer trick can work for a partial clog, but a fully collapsed conduit is a different story. The pipe walls have physically given way, so clearing the blockage doesn't fix the structure. You'd just be washing debris into the surrounding soil. Once it's pancaked, there's no real path for the new line to follow, which is why a fresh trench is usually the only permanent fix.
Ever try using a pressure washer to clear out the worst of it first? I blasted out a ton of clay and small roots that way, saved me a ton of digging. Might buy you some time before a full replacement.