I am looking forward to the pics of progress. I considered the Backroad but settled for my 1120. You might have an advantage over me with your triple crank. Also, you might like the Jones H bar. I love mine, disliking both drops and straight flat bars.

I have the TRP Spyre disc brakes on my mtb tandem and they are great, no issues with swapping wheel sets. My only suggestion is to go with wider tires (and tubeless, while you're at it) for off pavement touring. Lower pressure and wider tires makes a larger footprint and greater stability, in my experience, but I recognize that many run narrow tires on gravel and love them. All I can say is that I prefer 29x2.25 as a minimum, and my 29x3 plus tires are not too wide when the gravel gets inconsistent and chunky. The rollover is amazing. Your gravel and mileage may vary. Best of luck and keep us posted on your progress.