You’re being a bad influence again. I’m already too English-influenced as it is, and now you post this thing.
My ultimate, at least as of right now, would be something like my emergency randonneuse, but with thinner tubing (7/4/7 instead of 8/5/8) & a 73° HTA to increase trail, clearance for flat magnets, 12mm through-axles, insanely teeny seatstays like that English (yours?), and fly-by-wire (or less) mated to shimano mechanical brifters™ with the guts ripped out and replaced with shift buttons.
Not sure about disco vs. rim brake. I prefer rim brakes, but in 650b-land the light rims these days are all disc. So I’d plumb the frame for hydro cables and choose whatever Tarck suggests for a shimano-plumbing-compatable caliper.
Carbon fiber fenders, 1" threadless stem w/ a Herse-style wiper for lighting (no stupid stemtop switch – cars are evil but are often driven away by bright lights), carbon fiber stem cast from a 3t mutant pattern, carbon handlebars, carbon front rack, SON SL lazy-style dynamo, and whichever lights have the most shiny! that week.
Ultimate means cost no object, so I’d be showing up at Resist’s front door and throwing money at them for 650×28b & 650×35b tubeless versions of the Nomad (and then I’d give away 9940 spare tires from the 10k minimum order.)
I’d use a seatmast instead of a seatpost, matched # of spokes fore and aft, a 32/48 semi-alpine double (on an (imaginary) modern carbon low-q subcompact crankset) mated to an 11-speed 11-28 cassette on a White hub.
Maybe a C15, maybe a Soulor, but something narrow and hammocky any way.
And then I’d ride it about as much as I ride my other machines.