What am I doing wrong??

Hi all, every time I have made a loaf of sourdough, the inside is always a little bit doughy/gummy. I have googled what I could be doing wrong and made adjustments, but my bread is still turning out the same way. I always thought I was under fermenting, so I let the dough rise for longer this time and the result was the same. The photo is my bread I cut this morning! Any tips or advice would be greatly appreciated!!

Here is what I did:

-150g active starter, 350g water, 500g bread flour, and 10g salt

-Mix dough and let sit for 1 hour, stretch and folds 4 times every 30 mins, let sit and rise for 7ish hours

-Shaped and left in fridge for a few hours

-Baked at 450° with lid on for 35 mins, then with lid off for 15 mins

-Let cool completely before cutting