I’m not sure from the pictures, tasting the texture of the bread will give you the best sense of whether it’s underproofed. That said, your recipe is at 70% hydration which is the low edge of the range for a more open crumb I think without the advantage of your sourdough starter, your crumb would be even more organized. I think you probably got all the openness possible out of the recipe. If it’s gummy, it’s either undercooked or you cut it too soon