All the versions of the Raven were designed for Rohloff. The Sherpa is a very similar derailleur equivalent.
My feeling is that the Nomad is the more solid world tourer
That's pretty much it, though the differences between Nomad and Raven would depend on which versions of each you were comparing.
The Sherpa probably does get less attention, in the World of derailleur touring bikes it's one of many. The Nomad, particularly early versions, were as you say a step more towards the expedition end of the scale, a much smaller market. The Raven, particularly when it launched, pretty much had the UK Rohloff market to itself. Even now, when far more people know of Rohloff and there's more bike choice, Thorn still retain a large share of that market, when I see Rohloff bike it's more likely to be a Thorn than anything else.
Glad you like the Sherpa, if it suits you, that's as good as it gets.
EDIT - The Sherpa has been around long enough to get some reviews, have you googled it?