Seeking a more open, less dense crumb

Posted this same over on Reddit, hopefully it’s ok if I cross post here since I think it’s different people here on this forum

and might get some different ideas

I need some help with my sourdough. 

All my breads come out ok and taste pretty good, but almost always are too dense at the bottom, don’t have an even distribution of large holes, and don’t have the type of oven spring / height/ airy-ness I’m looking for.

I’ve also noticed my last few breads are hard to shape (dough is sticky) and hard to score (razor blade drags and cant make one clean deep score). 

My starter seems strong enough (doubling after 8 hours with lots of holes) feeding 2x / day for a few days before using and I’ve even tried augmenting with some instant yeast and get pretty much exact same results

Here’s the method I’ve been using: 

10pm night before 

• Feed starter with 10g starter, 50 KA whole wheat flour, 50 water
• Autolyse 400 grams KA Bread Flour with 300 grams water

10am 

• mix 100g starter and 10g salt with the autolysed dough 
• 4 sets of stretch and fold 30 min apart 

11 hours total bf time from time of mix to time of shape, but my kitchen is relatively cold (68 degrees) — this is shortened to something like 3-4 hours in the summer when it’s 80 degrees

9:15pm 

• preshape

9:45pm 

• shape 

10:15pm 

• put in fridge in bread basket with sealed plastic bag around it 

10am next day 

• bake in 475 degree f oven with enamel cast iron that has been preheated 1 hour. 
• 30 min lid on, 20 min lid off