Gaping hole

Too little heat under the baking loaf and possibly a lax degassing and folding problem, that when combined with a tough dry dough skin from too long in the banneton...  and no where for the expansion to go.  

 Q   How long was the dough in the banneton and how was it covered and where was it placed as it proofed?

I won't go over scoring, that was already mentioned on the other thread.  This might have saved the whole thing by deflating any large bubbles near the top of the loaf and making a way for the loaf to open under control.  Still the bottom could be closer to the heat source.

What is the oven set up?

Can you explain what you mean by... I think I overworked the dough?

It could also be that the dough had not proofed long enough at the time it was shaped and put into the banneton.   This would cause and trap a large number of large bubbles  just where they appear in your loaf with a tight crumb all around.  Docking or poking the loaf with a wet toothpick would release the trapped gas (so would scoring)  so that pockets of gas cannot expand and force the bottom of the loaf open.  The other cause of such bubbles is uncontrolled rapid yeast fermentation due to lack of salt.

into the oven so that the imaginary top of the loaf would be dead centre in the oven.  If that means moving the shelf down a notch, try it.   Once the oven is hot, wait another 15 minutes before putting the dough in.  Also look for a small narrow flat pan  you can put a cup of hot water into for steam.  Heat it up with the oven and shove to the side when putting the loaf in.  Pour in the hot water at the same time the loaf is loaded.  (just one idea of many)

The overworking part sounds like too much flour got worked into the dough during kneading.  Might want to look up a kneading video to be able to knead without much flour.  The kneading should be done with the mixing up of the dough and eventually when adding salt.  As a beginner, I would suggest adding the salt when mixing up the dough.  It helps you get a better grip of the timing and will reduce stickiness.  

Waiting on how long the dough lay in the banneton....  On another older post 12 to 18 hours was mentioned.   Too long dries out the crust and makes it tough.  How long is too long?  My dough never lies in a banneton more than 4 hours if that gives any ideas.  My average is less.   There isn't a law about how long to use a banneton but bannetons are designed to pull moisture out of the outer surface of the dough to form a tough skin.  To check the dough.   Feel the skin on your turned out dough, if it is very tough to cut or score, it has been in the banneton too long.  

"I've found leaving the dough in the fridge for the bulk fermentation means it doesn't rise well during the final rise, even when taken out of the fridge. Could this be due to another factor,..."

If you autolyse the flour and water with the sourdough culture, it is no longer an autolyse  (hydrating flour with water) and becomes part of the fermentation process so it counts either as a pre-ferment (adding more flour to complete the recipe) or part of the bulk ferment.  So be sure to judge it accurately.

If you want longer refrigerator time, put the dough to chill before it ferments too much, but shows signs of rising.  About a third risen.  Degas and fold before chilling leaving it in a covered bowl or container.   The fridge will then slow it down so that you can reshape the dough later allowing a final proof.  If the dough goes into the fridge very fermented and shaped, it will loose strength in the fridge as time passes eventually becoming ... basically....tuckered out starter with little rise.  

I'm thinking that your starter is perhaps not as strong as it could be and the autolyse you mention is helping the starter like a levain build.  When the dough was shaped it hadn't fermented long enough to be placed into the banneton so large air pockets are forming similar to those in the later stages of bulk fermentation.  They could also be trapped gas from shaping but I place my bets on rushing to the final proof stage.  The long final proof shows the dough eventually fermented but the crust was set into a firm shell.  With the heat of the oven, the pressure existing suddenly became big pressure forcing the inside dough to go somewhere, the weakest point, crack and ooze, another loaf is born.  

Perhaps holding off on the final shaping while the dough is refrigerated and proofing later in the banneton is all it takes.  I think that is what I would try first before too many changes.  Just change that one thing.  :)