Some random thoughts.
Possible factors:
>* Baking on a sheet pan rather than stone. This one seems
>promising, but will it impact oven spring? I would prefer not to
>lose bread volume, if possible.
Actually, the simplest solution would be to bake her loaves in a
bread pan. The challenge is to keep moisture on the outside of the
loaf during proofing and baking. You do need to proof a bit longer to
accommodate the fact that it the loaf can only expand in one
direction. But in the end, your volume can be the same as with a
free-form loaf. And you should still slash the loaves to control the
expansion. (I make my standard sourdough in loaf pans all summer
because, let's face it, a BLT or a tomato sandwich just isn't the
same when you bite into a thick crust and the slices squirt out and
scatter in all directions...
>* Baking at lower temperature for longer time. Would this give a softer crust?
I bake my loaf pans at a lower temp because I want to ensure interior
cooking before excess browning. (400F v. 440F)
>* Duration of final proofing. I have been retarding the dough in
>the fridge overnight before baking in order to increase flavor. But
>does retarding also result in a crisper crust?
I retard loaves in pans with no problems.
>* Hydration of dough. It seems like a dryer dough would have a
>softer crust, but I'm not sure. (I'm basing this on the extra-crisp
>crust of high-hyrdation breads like ciabatta, but I can't really
>grasp the physics or chemistry of why this should be so
A wetter dough often has a thinner crust, but it depends more upon
how long it is baked.
>* Oven steam. Would adding less steam result in a softer curst? Or
>is it the other way around?
Steam is more vital to oven spring than to crust thickness. Crust thickness
is more related to baking time.
>* Adding fats. I would prefer to avoid this one, but would adding
>milk, oil, or butter influence crust texture?
Fats are added primarily to give you a more tender crumb. But they
will also soften the crust.
>Obviously, a basic requirement is not losing the great flavor of
>traditional sourdough. If anyone has any expertise or insight into
>the best manipulation or combination of factors, I'd certainly love
>to hear about it!
It's simple: Bake your sweetie's loaves in pans at a slighly lower temp!
Cheers,
Bill