Crisper/Crunchier Crust

My experience with sourdough hearth breads is that after cooling and some time they have great shelf life but after a certain amount of time after cooling the crisp crunchy crust begins to become chewy.  I suppose the amount of time before this happens depends on many variables from the formula and the environment the bread is held.  I for one live in a temperate rain forest on the cost so lots of moisture in the air.  Is there a way to lengthen the crispness/crunchiness of a crust on a sourdough bread via formula or baking.  The bread is baked with steam for 18 minutes and finished in a convection oven for 20-25 minutes with added venting during the last 5-10 minutes.  The loaf is baked to a deep red with just a bit of "boldness" as a burnished crust is not desired. So baking longer/darker is not a solution.   

The loaf in question is a SF style sourdough with 15% pre-fermented flour (in a 12 hour 100% hydration levain) that is about 67% hydration.  Autolysed for 30 mintues.  Mixed to 78-80 deg F.  Bulk ferment for 4 hours with 2 stretch and folds.  Then it is shaped and retarded for 21 hours before being baked cold.  450 with steam for 18 minutes then 400 convection for 20-25 more.  

I tend to think this :"softening" of the crust is the nature of the beast but welcome any/all suggestions.

Thanks

Josh