85% hydration tangzhong sourdough bread - what went wrong?
It's hard to say exactly what went wrong, I've worked with plenty of high hydration doughs and sourdoughs so I have a few ideas though. I can see from your picture that you did get yeast action in your dough, with lots of holes and good structure, so you did something right, probably most of it. 

Shape

Firstly, whole wheat flour absorbs more water than white flour, so by substituting white for the whole wheat you increased your hydration. 85% is already very high, this would have added a couple of percentage points to that, which changes your consistency. Next time I'd reduce the water a small amount to compensate. 

High hydration doughs are tricky to work with, you need the right equipment at the right stages. Very high hydration doughs do not hold their shape, and tend to pool out if they aren't supported by something, just as you saw. That's normal, and a sign that perhaps you are a bit over-hydrated. Even if you had a proofing basket it's very possible this would have lots its shape right away. 

These doughs are also are extremely sticky, and easy to knock the air out of. When you put it in a container to proof overnight it would have adhered to the sides, if you were rough with it getting it out you may have bashed some air out of it. Be gentle, use oiled hands and an oiled scraper to free it up. 

Density

I think you had a good amount of air left though from your picture, I suspect a big reason for your dense loaf is that your oven was dry. Sourdough crusts form quickly in a dry oven and are very thick, I think what happened is that your crust dried out and prevented the loaf from being able to spring. Sourdough and high hydration doughs need to spring a lot, if they can't spring they get a consistency that is very dense and you get a raw dough consistency as you describe.

To resolve this you need a moist oven to start. Get your oven up to temperature and put a pan with plenty of hot water in it a good 15 minutes before you put your bread in to bake. Open the door, get it in quick, close the door most of the way and then squirt some water in with a spray bottle before closing all the way. Let it bake in your moist oven until it the spring stops and it stabilizes in size and shape for awhile, this is usually about 60% of your baking time, then remove the pan of water and finish baking in a dry oven. 

Safety Note: When you open the door of a moist oven keep back as the steam will billow out. It's probably not enough to burn you, but it certainly isn't comfortable! 

In Summary, reduce your water a bit to help improve the shaping, and bake in a moist oven to start to allow it to spring and fix your density problem.