A quick update, as you were all so helpful with suggestions above...
I've had a ride round the bike shop carpark on a Tripster AT, with SRAM gears. Very nice. The shifters felt comfortable to use, once i got the idea of pushing further for the other direction. But, I really don't think that 1* gears work for the range of uses I'm after. I'd end up compromised on either "quicker" road rides or rough hills, or both.
I had a go on current Shimano Tiagra shifters in a shop. I still don't like the ergonomics of pushing the brake lever over - it just doesn't suit my hands as well as the alternatives.
I also had a go on some electronic shifters at the Rouler Classic. They make a lot of sense for ergonomics, and the SRAM ones I tried had batteries that could realistically be carried as a spare. But they're deeply spendy so really not an option yet!
I think that puts me back into cable discs. Digging about on the forum, I spotted some positive comments about the Juin Tech brakes, which are cable operated by hydraulic in the brake - and sound fairly easy to adjust. So I've added them to the shortlist.
I had a go at pricing up the bits for building based on the tripster, XT gears and moderate shiny other bits. It came to a little more than I want to spend. Which makes the next thing good news
And, today I see that the new version Sonder is available as a frame. So I might pop them an email about chainlines and whether they're interested in a non-standard build.