Sour Dough Batard

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I've been baking with my whole wheat sourdough starter (my family fondly call it the monster) that I started two years ago from King Arthur's recipe online. I have pretty good consistency using the following formula that I developed over time.

1 Cup Starter

2 cups water

1LB 6.5 oz bread flour 

1TBS salt

I typically knead it for a good 15 minutes in my machine, then let it raise for 12 hours. I would then shape and final proof for 4 more hours. Bake at 500 for 10 mins with a tray with a cup of water in the first min and spray every 30 sec after twice. 

Sometimes, I get a darker loaf with tiny little blisters on the crust and great irregular holes in the crumb. However, I don't always get that crust or the holes in the crumb. The flavor is consistent though.

I would like to perfect this bread! Here are my questions.

1. What controls crust thickness? I would like a thin, crackling crust

2. How to get consistently get the gluten development that I see in French bakeries with irregular holes and translucent crumb?

Based on what I wrote here what would you suggest that I modify to achieve those results?

Thanks in advance.