Uneven Crumb

I've been baking sourdough bread for about the past 3 months now, and overall, I'm pleased with the results of each loaf - decent rise, good taste, good crust, etc. However, in almost all of the loaves I've baked, I've noticed that the crumb is very uneven - usually a few large air pockets near the top, and a dense crumb otherwise. Any advice for how to fix this issue? The recipe I'm using is based on the 'Beginner's Sourdough' recipe from theperfectloaf.com, but I've adjusted it a bit so that the total flour = 1000g:

80% bread flour

20% whole grain flour

78% water

2% salt

20% levain

Levain build:

40g starter

40g bread flour

40g whole grain flour

80g water

I feed my starter twice daily on a 1:2:2 ratio, 15g starter to 30g AP flour to 30g water. When making the levain, I let it rise for 6-8 hours until doubled. I autolyse the bulk of the dough for 1 hour, then add the levain, salt, and 50g of water. 4 hour bulk fermentation with 3 stretch and folds, then preshape with 30 minute rest, then shape and overnight proof in fridge for 16 hours. Baked in stone bread cloche at 475F for 25 minutes, then uncovered for 25-30 minutes.

Any advice is greatly appreciated!