Loaf failing to hold shape after final proof

So I've got into sourdough baking over the last couple of months and while I'm very happy with the flavour and crumb of my loaves in general, there have been a couple of stumbling blocks when it comes to aesthetics and rise. You can see the full details of my method down below. 

Everything appears to go fine until step 11, where I attempt to shape the loaves. The dough reacts well to being pulled and stitched (similar to this instagram video: https://www.instagram.com/p/BP-hH35gyLi/) and gains some tension overall, but almost instantly begins to relax and spread out, unlike that video. It certainly doesn't retain the nice rolled shape, and I sometimes even repeat the same pulling and stitching maybe 3 times since the shape just keeps disappearing. Either way, any semblence of shape disappears when the loaf is put into the bannetons anyway, and it will always emerge very soft and delicate, with no tension and will immediately spread to fill the bottom of the casserole dish. This then makes scoring very difficult, and while there is a decent oven spring and the loaf doesn't come out dense and flat, it definitely doesn't develop a nice ear and isnt quite as tall as I would like it to be. Sometimes I have tried taking the dough out of the bannetons and then reshaping them once more to gain some tension, before plopping it into the casserole dish and then scoring is a little easier, but then the loaf often ends up with a bit more height, but is certainly a little smaller in volume overall from being degassed a bit when reshaping. 

My thoughts on how to improve my loaves are:

• Improve my kneading/folding techniques in steps 5-8, but I'm not really sure what I could be doing wrong when looking at videos of people doing their folds. I tried doing a longer and more basic kneading method in step 5, where I just work the dough more than just the counter slapping, and for up to 10 minutes, but without much difference.
• Use less wholemeal flour in the starter/levain/overall recipe, since the wholemeal could be disrupting the gluten formation.
• Lower the hydration to 70-75% levels. I'm hesitant to do this since I know people can get tension out of even higher hydration levels than 78%.

Any ideas or help would be very much welcome!

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My recipe is:

810g Allinson's Very Strong White Bread Flour (14% protein)
90g Allinson's Plain Wholemeal Baking Flour (12% protein)
680g water
18g fine salt
175g levain (100% hydration)

The result, by my calculations, is a 78% hydration dough.

Method is:

*N.B. Dough is kept on a 32°C/90°F heated surface at all times until put in fridge*

1. Mix levain, wait 3 hours/until healthily doubled or more in volume.
2. Mix all the flour with 580g (85%) of water, leave to autolyse for 1 hour.
3. Mix 18g fine salt with the remaining 100g water. 
4. Add levain and water/salt mixture to autolysed dough. Combine by kneading until water and levain are incorporated. Rest for 20 mins.
5. Lift and slap dough onto counter for 5-ish minutes, until dough is smooth and not watery at all.
6. Return to bowl, cover, and keep in warm place.
7. Every 15 mins, lift dough away from side of bowl, lift up and stretch without tearing, then fold back down on itself either once or twice depending on how slack the dough is at the beginning. Work around the whole edge of bowl, maybe doing 4-5 folds in each repetition. Repeat 3 times (total 45 mins).
8. Every 30 mins, repeat folds in previous step. Repeat 3 times (total 1h30m).
9. After final fold, leave for 1h30m.
10. Turn out dough onto work surface and split into two.
11. Shape dough into batards using some combination of pulling/stitching etc.
12. Sprinkle bannetons with rice flour. Place dough into bannetons and into the fridge overnight.
13. Preheat oven and cast iron casserole dish to 250°C/482°F for 1 hour.
14. Remove banneton from fridge, and place dough directly into casserole dish. Score with sharp razor blade.
15. Place casserole dish with lid on back into 250°C/482°F oven for 20 mins.
16. Remove lid and bake for another 20-25mins on 230°C/446°F.
17. Remove loaf from dish and leave to cool before slicing into it.

Starter feeding is done once a week. I discard all but maybe 30-50g of old starter, then dilute that with 100g boiled water that has been left to cool, and then adding 100g of Allinson's Plain Wholemeal Baking Flour (12% protein).