100% Whole Wheat sourdough, even crumb?

Hello all - 

I've been on a very long quest to achieve open/even crumb whole wheat sourdough with many many trial and errors and experiments. 

I feel like I'm finally getting somewhere with it, but the crumb still seems uneven, often tight in the center, and very open along the sides as seen in the pic. Any tips on why that might be happening and how to remedy it?

Here are the specs for this bread - 

83% hydration, 415g High Extraction Flour (83%), 60g Whole Wheat flour (12%), 25g Whole Dark Rye (5%), 10.5g salt (2.1%), 75g stiff starter (15%)

I soak the flour overnight in the fridge with the total water minus 25g, and prepare the stiff levain. 

The next morning I mix in the levain with Kitchenaid mixer at speed 3-4 for about 5 minutes until passes strong windowpane test, let sit for 30 min. 

Mix in salt with remaining 25g water with mixer for about another 5 minutes until fully incorporated and silky smooth, window pane test again. 

Then I do a 4 hour bulk at 27C with 2 coil folds 1.5 hours apart. 

Gentle preshape to bench rest for 30 min. Then gentle and simple final shaping to a 15 minute rest before going into the fridge for 18 hours. 

Preheat Lodge Combo-Cooker at 500F for 45 min, then bake with lid on for 20 min at 450F, then another 30 min at 420F with lid off. Going for a longer/lower heat bake to fully cook the dense whole wheat dough without burning the crust too much. 

I also don't score the bread to let it open naturally in the oven (I final proof it seam side down, so when I bake it, it is seam side up), which seems to be a helpful technique for whole wheat sourdough. 

I am overall very happy with it, it tastes great and feels light and airy, but I can't help but be frustrated with this consistently uneven crumb distribution! Any help/advice would be much appreciated!