Dough extensibility and prefermented flour

Basically what factors can lower excessive dough extensibility? Lower or higher starter amount? Starter more watery or less watery? Starter before or past its peak? 

My dough seem too extensible after bulk fermentation. I usually use 20% of flour weight with starter, which has 100% hydration. So for example for 500g wheat bread flour[13% protein] i would use 100g starter. The overall dough hydration is 70%. I feed my starter with cake/pastry flour since it has lots of sugar and its pretty cheap and universal[good for pizza dough too]. The bulk takes 6-8h. My starter usually almost doubles [2/3 size] after 4-5h when taken out of fridge and feeded. I use scraping method, so i usually leave 50g of starter in the jar, leave it in fridge for next feeding and baking[i bake each weekends]. It has fruity smell. The bread it makes taste good with moderate sourness. But the dough is kinda slack. I don't do autolyse as the bulk is already quite long. I mix the dough, do some slap and fold for few minutes until the dough smoothens out, then i do 4 coil folds each 45 minutes apart. I bulk ferment at 21*C. 

My ideas:

1. More/less kneading? More/less coil folds?

2. Less/more starter? 

3. Get starter more active?

4. Get starter more/less watery?

5. Ditch the cake/pastry flour for feeding starter?

I didn't have such problem with yeast dough.