Sherpa owner here, but no good to you as i'm in Yorkshire.....Have you found the Stanton owners group on facebook? I went through the same process as you, trying to get the sizing right, and managed to find a few local owners happy to oblige. If its worth anything, i'm 5'10" and ended up comfy on a Large (19"). Great bike, suits me sir!
Good luck!

I’ve had two one a 17” the other a 19”. I’m 5’10” also with 32” inside leg and found the 19” a better fit for me. Ran it with 140 pikes and 740mm carbon bars and a 50mm stem really wish I’d never sold it.

Cambridgeshire if that's any use.