Hi Andy,
Have you considered an eXp? Before the Rohloff versions, they were very like the derailleur-mech Nomad Mk1 as I recall, and used 19mm seatstays and larger-diameter, thicker-walled tubing than the Sherpa in places. Some were built in Thorn's frame shop by noted builder Kevin Sayles and are very fine bikes indeed. Depending on geometry, you might even be able to use a slightly larger marked size and come out essentially the same. Pic of one below.
Just a thought to keep in mind, as it might increase your selection pool.
Best,
Dan.