Thanks all for the responses.
In the ultimate anti-climactic resolution to this question, probably for a bit, I found an old Vittoria Cross Ranger (I guess this evolved into the Randonneur?) 700x38 and threw it on the back, and double checked that I'm still good to go with my Marathon Supreme up front.
Findings, of note:
1. There's quite a bit more air in a 38cm tire vs a 35- bumps that I'd feel a tiny snap to ride over with 35s have sort of blended into the white noise of the road with this new rear tire (with lower psi)
2. Heavy tire = slow to get going = harder to slow down, even if just by a little.
TLDR; Jan and Petersen are right
Anyway- when it's time, I'll likely have a hard time deciding between the Gravel Kings and the Barlow Pass...
Tim