I ride 25mm tire most of the time and when I get on 28mm now they seem enormous. I'd probably go with 28s if my Cervelo had more room for them, but much bigger than that and I'm sure I can feel the inertia of the heavier tire and I seldom go on ground that needs a really wide tire and the rolling resistance benefits above 28mm are diminishing. The last puncture I had on my 25mm ultra Gatorskins was 2 years ago.
28 mm is as wide as I go these days even on my heavy touring bike. I don't like to be right at the limit of what the frame takes, so think 25 mm is about right for me for the very light type of touring that I do. It is nice to have a bit or wobble room in case a wheel gets knocked out of whack.
I started the ST in 2012 with 23 mm gatorskin tires and when they wore out switched to 25 mm gatorskins. On the rough Texas chipseal the buzz was reduced a good bit with the 25 mm tires, but the 23 mm were not that big of a hardship.
On the geometry and frame material choices... I find my aluminum and carbon fiber road bike supremely comfortable. I have ridden centuries on it and on my heavy touring bike and found the road bike to be more comfortable. I try to duplicate the aggressive position/posture of the road bike on the heavy touring bike. I'd definitely ride something like the OP's Roubaix on even long camping and cooking tours as long as I am packing very light.
Given that the OP already has the Roubaix and wants to do short credit card tours, it seems like it would be a very weird choice to buy a heavy touring bike. The Roubaix actually sounds about as perfect for the task as I can think of.