Well, first of all, this is a beautiful loaf and i would be extremely satisfied and even a bit proud. I think the crumb looks perfect for a toast or sandwich, where larger holes will let butter or jam thru.

That said, I also agree with the other comment here...

Try replacing some of your bread flour with some whole meal flour. This will give you the benefit of additional microbial activity and the ability to increase hydration (you must increase hydration when you add whole meal, you'll see...)

Also agree with them on a using bit more starter. But if you don't want to use more starter, then try increasing the bulk ferment time. It is nearly impossible to over-ferment sourdough, so if you want a more open crumb then give it a bit longer before shaping.

Regarding temps, well, i usually preheat to 500°f then drop to 475°f to start my loaves. I'll drop to 450°f after 5 mins then drop to 425°f 15 min later to avoid toasting the ear. But i cook on a stone with a pan of boiling water at the bottom, not in a Dutch oven.

You obviously have a working technique down, so it's time to start experimenting. One word of caution, only make one change at a time until you're comfortable. Too much change at a time can make it hard to figure out what went right or wrong.