Double Fermentation method: troubleshooting, please?

There are two aspects to the openness of the crumb. One is how expanded the loaf is.  You can have a very airy, open crumb with all the cavities small and even in size.  At first glance you might think that the crumb is dense but it is not.

Or you can have many cavities that are visible large and irregular in size.  I've seen many pictures where the crumb looks like that but it's actually quite dense between large openings.  Very often those kinds of openings are the result of shaping techniques needing improvement.

Or you can have crumbs anywhere in between.  The very best of these very open crumbs show a distinct, lacy network - see the lead picture on this blog post:

https://newsletter.wordloaf.org/p/the-biggest-secret-that-is-not-secret

I don't think the loaves in the video you linked to have achieved that degree of open crumb (and neither have I).

So, about shorter bulk ferments.  I don't positively know; I have to speculate. Let's imagine going to the other extreme.  You bulk ferment for way too long, though not so long that the dough collapses.  Most of the yeast nutrition will have been used up and the gluten may have relaxed too much, or even have started to degrade.  When you go to shape and bake, there won't be much vigor left to expand during baking.  You might end up with a good loaf volume but you won't be seeing many larger cavities, and your scores will have opened only a little.

Obviously your loaves aren't anywhere close to that extreme.  But you can see there's a spectrum.  Larger cavities are easier to expand than small ones (it takes less pressure), so early on there should be a larger portion of visibly larger cavities.  Early into the bulk ferment, but of course after it has started to visibly rise, the rate of gas production will be large and that gas pressure can support a larger, more fragile dough structure. There is simply more reserve oomph in the dough.  During the bake, this active dough will generate more lift during the first part of baking.  A larger percentage of rise during baking should, it seems to me, have the potential for creating a more irregular set of openings, which is what you are thinking about.

Or too simplify the picture, you develop the gluten and do stretch and folds to create a scaffold.  The fermentation fills the scaffolding to proved size and rigidity.  It's like filling a football with air.  The pressurized air provides the rigidity.  The gluten scaffold provides the basic structure and the gooey starch traps the gas.  Once that's all in place, you can move ahead with shaping and baking.

At this point I hope that someone else who knows a more definitive answer will chime in.

So why would anyone go for a long bulk ferment?  Partly because that's what most of us have been taught to do, so we're used it.  Partly because some of us like a finer-textured crumb for most of our breads (I'm one of those).  And in part because the longer the fermentation, the better the taste (that's me, too).

"Let them get bigger".  It doesn't usually work like that for me. Longer ferments and proofs usually end up with a finer, more even crumb (yes, well expanded, see my comment above). There's more to it than that.

One would think right before baking. Enjoy!