What am I doing wrong?

Dear Bread Experts, I need help. I've been baking the tartine recipe for a long time (200g levain, 750g water, 900g white flour, 100g ww bread flour, 20 g salt) and have been making glorified pucks. Recently I found a bulk fermentation guide and realized I have been significantly under fermenting my dough since my kitchen is very cold. I'm now utilizing temperature to estimate fermentation time. This batch bulk fermented for 8.5 hours at around 74 degrees. However, the loaf still seems a bit flat. Do you think it means my starter is weak or am I still underfermenting? I appreciate any assistance. Signed, A bread baker at her wits' end