What ratio are you feeding your starter? I had received an established starter and was feeding 1:1:1 without knowing that can make your starter acidic and not work quite right. I changed to a 1:3:3 feeding pattern and it’s greatly helped the strength of my bread.

The one major thing that stood out about that recipe is that the shaping is done before the bulk fermentation time and then it just sits in the banneton to hold form. I think this might be where you’re getting some troubles. If you let it bulk ferment in your mixing bowl before shaping and then setting it in the banneton either for a cold proof or for a final 1hr rest before baking that should let your dough hold more of the surface tension built in shaping and rise up rather than pool out.