Is a 68 degree headtube angle and a 68 degree seat tube angle (measured from horizontal) about the right geometry for a loaded-touring bike? If not, what would be?

That's far from modern typical, which is often 72/72 is probably closer to normal. That said, if you are willing to engineer the fork and stearing, you can go as low as 68 on the head tube and gain longer wheel base and tailor stearing and load handling. The seat tube I tend to build from fundamentals of my postion, and the seat post and saddle conbination that makes sense for my kind of riding. That is normally in the 71-72 range for me.

Waterford 1900 Touring Geometry

Waterford T-14 Touring Geometry

Waterford ST-22 Sport Touring Geometry

68° seems a bit extreme to me.

Old 80's stump jumpers make decent loaded tourers, they have slack angles,
71/ 68 perhaps 'Repack' bombers was the model, Tom Richey's bikes were the ones Synyard copied.

thing you gain when the seat angle-setback is a lower angle is weight bias, less weight in your arms.

look back at old TdF race bikes they were pretty low angle long chainstay designs, most roads were un paved then.

If the wheel choice is 622 - 35 and rate of travel is your travel priority , adopt a race bike 73 /72 main triangle.

of course Size of frame is another thing, angles is part of getting fit better.