After experimenting quite a bit, obtaining an airy crumb is a two-fold process. I use a stand mixer to mix my ingredients. I run it on low for one minute followed by running it on the next notch up for three minutes. I then lest the dough rest for 30 minutes before doing a series of stretch and folds with a slightly wet hand to develop the gluten. I do three sequential stretch and folds, thirty minutes apart. I let the dough rest on the counter for another hour and half after my last stretch and fold before placing the dough in a bowl that has been lightly oiled with olive oil and then coated with rice flour, which prevents the dough from sticking when I flip it over onto parchment paper for baking in a dutch oven. Retarding the fermentation in the one's refrigerator before placing the dough in heated dutch oven is paramount to getting good oven spring. One's baking oven and the dutch oven has to be heated to at least 450F, 500F is better. The dough goes into the dutch oven on parchment paper and then the lid is attached before placing it in the oven. The beauty of using a dutch oven is that it traps steam. The bread is baked with the lid on for 30 minutes and another 10 to 15 minutes with the lid off.
Now, seriously open crumb requires a high hydration rate. We are talking about at least 70% hydration (i.e., 70 grams of water per 100 grams of flour). I do not like working with high hydration dough because it is very sticky, but high hydration combined with what I wrote above will result in a bread with very open crumb. I find that a hydration rate in the 60 to 65% range to a good compromise between being able to handle the dough and get a nice crumb.
With that said, the starter I have been feeding for 9 months has now reached a point where the sourness is low, but the rise is amazing, which means that yeast are winning the battle. I suspect that the dominant culture at this point is a strain of saccharomyces cerevisiae because the culture now smells like beer after being propagated. There has also been a noticeable drop off in bacteria-related aromas. In order to limit discard, I have only been keeping between 50 and 100 grams of starter. I add new flour (70% unbleached bread flour/30% whole wheat) the day I am planning to make dough. I only add enough to leave me with between 50 and 100 grams of starter after I harvest my crop for baking. The starter goes immediately back in to the refrigerator until the next propagation. I usually propagate starter at least once a week. This culture is now so strong that adding 100 grams of mixed white/wheat flour and 100 grams of filtered water will double in two to three hours at 65F. It behaves very much like dry activated yeast, but the flavor of the finished product is better.
By the way, I posted this image on another thread, but here is one of my early successes. Getting an nice ear to form after scoring with a bread lame is part of getting good oven spring.