Mission success on making a decent sourdough at the restaurant, last two batches turned out very nice (and consistent). Previous crumb issues were due to lacking fermentation from a weak starter. Began feeding it twice a day and now it doubles in 3 hours instead of 5.

Also made sure to use warm water in the dough to kickstart the fermentation and mix some of the water into the starter to make it mix more evenly into the dough, avoiding uneven fermentation.

Furthermore, I've begun 'over-mixing', doing 5 minutes on low speed and 5-6 minutes on high speed in the mixer, letting the dough rest a few minutes when it starts to tighten too much. Makes it much easier to work with during the bulk ferment.