My old Colnago has the sheathed rear brake along the top tube. It never gave me problems. It uses clamps. For a while it was on the side, but I centered it about a decade ago.

I've ridden other bikes with bare cables along the TT... also never any problems.

Internal cable routing looks sweet... but perhaps for touring, the simplicity of accessible cables would be a benefit.

I agree with others. You are putting an awful lot of thought into the perfect touring bike.

I'd encourage you to either use what you have, or find a cheap Craigslist Special. Put a few days on the road, then decide what you do or don't like about the bike.