Sourdough starter that is active but produces dense loaves

My sourdough starter doubles reliably after feeding and smells pleasantly tangy. The loaves still come out squat and dense, with a few enormous holes surrounded by tight crumb. I'm using bread flour and folding during bulk fermentation, but judging when fermentation is complete is guesswork in my cool kitchen. Would you focus first on bulk rise, shaping tension or final proof? The starter looks lively, so I suspect my timing more than the culture.