My Rambouillet I think fits the bill of what you want to do with your new bike.

It's suitable for fast road riding, with the right tires and wheels it can go off road, and I used it to pull a trailer last year in Nova Scotia. So geometry wise I think you can get all the things you want in a custom.

Gearing - why switch from your friction shifters? Maybe go to bar ends. We went up one hill in Nova Scotia that I could barely get up with the trailer (70 ish pounds) and I have a a 24 inner chain ring and a 27 in the rear. I guess if you're never going to haul the kids anywhere hilly.

Wheels - I'd never go off road with the Mavic Ksyriums I have. But my first ride with my hand builts we went wandering off on a new fire trail.

Good luck and happy planning!

V.