Need Help on Crumb, Last 5% of making the perfect Tartine Country Loaf

I post on rare occasions and mainly follow everyone's great work on this site. I need help achieving the last 5% of the tartine country loaf-- a good interior crumb. Or maybe it's more like 50% of the importance haha.

Currently, my crumb is very uneven where some areas are chewy, some are light and airy, and other areas have too much bubbly airholes. 

I was following the Tartine loaf recipe pretty religiously but I was getting even worse interior crumb than what I'm getting now. The big difference now is i'm first mixing in a stand mixer until I get a windowpane effect and no autolyse.

I've tweaked my approach to include some tartine elements and other elements similar to this method: https://www.youtube.com/watch?v=2FVfJTGpXnU

My ingredients are:

• I've used King Arthur and Bob's Red Mill. But the current flour I've had most success with is actually a mix of the inexpensive white unbleached AP Ceresota Flour and Bob's Red Mill Wheat Flour. I'm judging "success" as having a more active, robust, and floating starter. 
• I've also started experimented with adding 3-5g of Bob's Red Mill Flax Seed Meal to my starter and it has made the starter more active.

In terms of maintenance:

• I bake about ever 2-4 weeks. I keep my starter in the fridge and feed it every Sunday.
• When I plan to bake, I get the starter out 3 days in advance of baking and start feeding it daily to get it back to a good activity level. 

My current approach is as follows:

• Mix 75g starter, 75g water, 80g flour 8-12 hours before the bake.
• Only move forward if the starter is at least partially floating. I've had the most successful loaves when a teaspoon size ball of the starter floats atop the water and never sinks. I believe that's always the goal.
• The rest of recipe for one loaf is:
  • 160g leaven
  • 230g water
  • 350g white flour
  • 50g wheat flour
  • 8g salt
• I first mix the leaven in the water until it looks as if it is dissolved into the water for a creamy, cloudy liquid mixture.
• Instead of autolysing like the tartine recipe and waiting 30 minutes to add the salt, I instead add all ingredients to my kitchenaid mixing bowl at once. 
• Unlike the tartine country loaf where I was not using the mixer at all, I currently use the mixer to beat the dough and I run it for about 12 minutes on medium-low speed until I achieve a windowpane effect with the dough. 
• I then let it rest for 3 hours at 78 degrees and fold use the tarting folding technique every 30 minutes during this time.
• I then shape the dough with a structural stretch, place it into a bread basket, and let it rest in fridge 8-12 hours.
• I take the dough out of the frigde the next morning and let it rest on the counter for 1 hour. 
• I then bake in a terra cotta/clay vessel. It's essentially a terra cotta dutch oven. The clay vessel is preheated in a 500 degree oven and is hot as soon as I place the dough in it. I slice a line in the top of the dough and spritz with a handful of sprays of water to add steam before I put the top on the vessel.
• I bake for 30 minutes in the vessel. Then I take the lid off the vessel, reduce heat to 475 degrees, and bake for another 27 minutes.
• I let it rest a couple hours before cutting into it.

Again, my biggest issue is having an uneven interior crumb where some spots are airy and perfect while a majority of other spots are chewy and too dense/doughy. 

Here are some pictures of one of my recent, more successful bakes:

Thank you for considering to add your expertise and input! Much appreciated TFL people!