Sourdough

Hi I have an issue after bulk fermentation, I divide preshape, then shape into boules or batards. Starts off okay then the gluten seems to give up and the dough tears and flattens?

Could I be using too much starter? Last recipe was 460gm starter (50/50) 630gm flour, 400 water.

I mix till dough comes away from the sides (3-5min) then use a dough hook for 3-5min till i think enough gluten is developed and it looks glossy (remains sticky to the touch)

Proves well (doubles in 3-4 hours) Cheers for any advice.