GM!
Even though it turned out a little flat i still think this is going to be great. Happy with my first time results. 
Discussion
That's a great looking loaf.
Sourdough typically ends up a bit flat in my experience.
Maybe, it was super tasty and had good crumb so no complaints here.
What kind of flour are you using? I have found that higher gluten holds structure better and a very active starter can also contribute to oven spring.
I switched from a high gluten flour (Grain Craft's Power Flour) to Cairnspring Mills Trailblazer bread flour and found that my loaves are baking flatter - I judge them as to whether they 'break the plane' defined by the opening of my 5 quart cast iron dutch oven.
I bought some bulk vital wheat gluten at the co-op last weekend and am going to experiment with boosting the gluten of the trailblazer flour, which is locally produced in WA state and does not use glyphosate as a drying agent. The power flour has great performance for bread and pizza, but it is a national brand and may not be as clean.
I'll try to update with results.
#foodstr
I was mostly following a tutorial by a german guy of YouTube called "the bread code". I got some wheat flour from a local mill with 13% protein content.
For optimal hydration he had a smart trick where you make 5 sample doughs with 50g flour and then 60, 65, 70, 75 and 80% hydration. You just mix the flour and water and wait 15 minutes and then see which has the best window pane. Mine was at 75% but i went with 70 cause lower should be easier.
I think what my flaw was that i stopped adding dough strength during the last 4 hours of fermentation, so my first change will be continuing coil and folds for longer and see if that fixes my issue.
Higher hydration doughs are definitely trickier to handle.
I read Ken Forkish' book and then hacked his method to suit my lifestyle - mainly lower hands-on time / less fussiness. I typically let it go for 24 hours and manage temperature during fermentation & proofing. I have to adapt to the season.
Today I'm going to use the cooking water from a batch of chickpeas/garbanzos to boost protein content.
Happy baking!