Better country bread flavor - what to try next
I am a pretty new baker, with 6 bakes of Tartine Country Bread so far. I'm not consistent yet but it's improving, and my better bakes taste ok, with a glossy crumb and decent structure, and really really good by my previous standards. But, I'd like more flavor. I don't want it more sour, or with more whole wheat tang. I'm looking for a more complex flavor without more sourness. Do I try extending the bulk rise? Or try different flour blends? Or something else?
I usually make a young leaven following Chad's instructions (1 tbsp starter, 200g 50/50 flour mix, 200g water) and use 200g after about 7-8 hours. It passes the float test after about 3-4 hours, but 8 hrs matches my schedule. I'm using King Arthur AP and King Arthur White Whole Wheat. I've tried 10%, 15%, and 20% whole wheat. More whole wheat helped, but still not the flavor I'm looking for. My kitchen and ingredients are all in the mid-70s, and my dough usually ends up around 77-78F after the final mixing.
My last 3 attempts:
I tried letting the bulk rise and final proof go as long as possible at room temp, about 10 hrs total. The loaves completely filled their proofing baskets, and climbed the sides of my 3.2qt dutch oven. Great crust, good flavor, but they didn't really taste that much different.
Next, I tried a regular bulk rise, and then overnight proof in the fridge. Yuck. Those loaves ended up too sour for my taste, definitely not what I wanted.
Next, I tried an overnight bulk rise in the fridge for 12 hrs, then proofing for 3 hours - a little under-proofed compared to the last one, but I wanted them to fit in the dutch oven. I loved the dough I got from this method, final shaping was a dream of pillowy softness. The flavor was good, but still a little white-bready for me.
From what I've read I think the flavors I want happen in the bulk fermentation, so I'm thinking I may try colder water and an overnight bulk ferment at room temp, vs in the fridge. But I'd really like some advice and suggestions from more experienced bakers out there.
That was a wall of text, so I'll stop there. Thanks for reading all the way to the end.