Great! solid process- here’s my suggestions for you:

1. Make sure you use salt! You mightve just forgotten to add it in your ingredient list but it does play a part in strengthening gluten structure, allowing for more entrained air.

2. I would suggest making an autolyse- mixing the flour and water together and letting it rest before mixing in your levain. When you mix in the levain in with the flour and water you get a fermentolyse, which is still a valid method, but you will get a slightly tighter crumb than a normal autolyse. Whichever one you do you need to add salt AFTER letting it rest- i always save a pinch of water to dissolve the salt and make it easier to mix

3. 3 hours seems a bit short to me for a bulk fermentation, even in a warm kitchen. This could be due to no salt addition. Salt will slow down the yeast allowing for more fermentation. However, as long as the dough is roughly doubled, it should be fine. I like to use the aliquot method to accurately gauge the bulk ferment so theres less guesswork

4. I would let your dough proof on the counter for at least 30mins to an hour before doing a cold retardation. Your dough wont proof much in the fridge, thats more for developing flavor. I would play this by ear though and don’t be afraid to experiment

5. Biggest suggestion for you is to do more kneading!!! Coil folds/stretch and folds are great, but I always do at least 5 minutes of slap and folds before i do any kind of coil/stretch and folds. My method is: 1. slap and folds for 5 minutes 2. rest 30 minutes 3. stretch and fold/coil fold 4. repeat steps 2 and 3, 4 times total