No, don’t buy a stretcher. Just keep practicing and your fingers will adapt the more you use them. You might never get that full reach unless you have massive hands like Josh does, but you’ll get closer and the shifts will get smaller, and you’ll develop muscle memory for the shifts too.
For reference I’m just starting Module 14, working on learning Hound Dog for the 50 Songs Challenge, and that song is all about bouncing between the 3rd, 2nd, and 5th frets. I cannot do it without shifting.
That all said, I’ve been doing this exercise as a warmup for a few weeks, and I think it helps:
