WW Bread Gummy Problems

My first thought with gummy bread is that it is underbaked. If you are pulling it straight from the fridge and putting it into the oven it is going to take an extra 10-15 minutes for a large round loaf like that to cook all the way through.

I actually think the loaf is showing nice browning on the bottom and top and your additional info indicates that you baked it for a good amount of time, so I don't think it was under-baked.  I usually do 25 minutes covered at 500, then 20 minutes at 450 with the lids off (for an approximate 750g loaf).   I nearly always bake straight out of the fridge after an overnight cold retard, into a nicely pre-heated oven at 500F, so I don't think that is an issue either, especially since you are only doing a cold retard for 2 hours, which would barely cool down the loaf.  Your crumb looks pretty good actually, so a couple of thoughts sprung to mind.  You could reduce your overall hydration by a good 5% and see what that does for you.  Also, I always leave loaves in the oven once bake time is over...turn off the oven and open the door, letting the loaves 'bake off' for an additional 10 minutes.  You also want to let freshly baked loaves rest for at least 4 hours if not longer before cutting into them--a certain amount of internal moisture remains immediately after baking so you want to give it time to evaporate.  These last steps can help alleviate some of the gumminess issues.

As an aside, I'm assuming your processes above are based on what the dough is doing rather than rigid times?  So, for your bulk ferment, assuming that is for a certain percent or rise rather than just 'for 12 hours'.  Not sure if you have flexibility in your baking schedule, but you could try a shorter bulk ferment (once you get adequate rise) and then a longer cold retard--you might get more oven spring that way, which could also open up the loaf and decrease any denseness or gumminess.

Just a few thoughts...hope you find something helpful.  Happy baking!

In place of stretch and folds, you might consider a long coil fold followed by a lamination as is described in the youtube 

https://www.youtube.com/watch?v=PGTYDQQSd54&list=PLJO9qm7b5OdNC2fp6fk_SR_QQAGX5D801&index=4

see time 11:12.

There are also other extra steps like sifting the whole wheat flour and regrinding the larger parts 2-3 times more (do not discard any of the grind). 

Bulk ferment: 5-6 hrs at room temp, 73F?

After shaping the dough, overnight retard in refrig [the dough is in banneton or loaf pan].

I very carefully followed the steps in the above video.

After getting better at dough handling and knowing when to end bulk fermentation, the gumminess is much less (almost gone) in  90%+ whole wheat lean sourdoughs .

Keep trying!! The nutrition and texture are worth all the work and steps.

Definitely not overproofed. In fact, just from the pictures your crumb looks very good. Properly fermented. You could incrementally push the fermentation further (the final proofing further) to see how much more the dough structure will take. It is best to proof at room temperature for this, not in the fridge.

One possibility is that you're adding a significant (20% pre-fermented flour) amount of sourdough discard. Depending on the protein content of the bread flour used and how long the discard has been stored, you could in effect be adding a lot of degraded gluten to the dough. I can't point to a mechanism that would then promote gumminess, but if you find your loaf is flatter than you like, adding a lot of discard could be the reason.

Something else to try is upfront gluten development after mixing and before the stretch and folds.