How to get more oven spring?

I am looking for some advice on how to increase oven spring in my sourdough. The photo is of my most recent loaf. I used The Perfect Loaf Beginner's Sourdough Recipe, the only alteration was that I replaced the 51g rye with more whole wheat as a family member can't eat rye. I've made bread with this recipe a few times, this is the best so far which may have been because it was a warm day, or because I've just started using a dutch oven? I think the main problem is that when I put the bread in the oven, it spreads out and doesn't keep its shape. It doesn't seem to rise much. I don't know if this is a problem with my shaping, proving, starter, or something else. 

I would also appreciate any advice on how to get a thinner crust, I used to get a thin crust baking in a stainless steel pot but since I started using the dutch oven it seems to have improved the oven spring a little but the bread comes out with a very thick crust.

Any tips on how to improve my sourdough would be really appreciated! I hope I've given enough information, please ask if I've missed anything.