Is this underbaked?

I was trying to convert a yeasted raisin cinnamon loaf recipe to one using a starter. Either my math was off or my starter was having a really bad day...

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Original recipe: 

• 180 g whole wheat flour
• 180g all purpose flour
• ¼ tsp yeast
• 1 ½ tsp salt
• 1 tbsp cinnamon
• 350 ml room temperature water 
• 50g walnuts
• 50g raisins

Modified recipe:

• 1/2 tsp sourdough starter (100% hydration)
• All other ingredients same

What I did: 

1. Mixed flours, salt, cinnamon. 
2. Added in walnuts and raisins.
3. Dissolved starter in water and added it in
4. Bulk ferment at rtp (28 degrees) for 12 hours, with 2 S&Fs in the first hour
5. The next day, preshape dough and bench rest for 20 mins
6. Pre-heat oven to 250C. Place dough in loaf tin and bake for 60 mins
7. Slice loaf after cooled.

Issues faced: 

1. The dough was too wet throughout and I was unable to develop tension adequately during preshape. (I did not flour my work surface, only sprayed it with water to prevent sticking.
2. Although placed on top rack of oven, the crust failed to brown, even after an hour.

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When sliced, crumb was VERY gummy. After slicing a few slices, I placed half of the uncut loaf back into the oven in hopes that it will bake through fully (to no avail). Also, the few slices that was already cut was placed into the oven to dry. However, it only made the surface crisp but the interior was still gummy. After these slices have cooled, they hardened into bricks. I don't think they can even be repurposed into croutons. :(

 

Where oh where did I go wrong? P.S. I've baked the yeasted version before and it turned out great!