100% whole wheat hole structure

First post here.  Would love some pointers on getting my 100% whole wheat to open up a little more.

Just made my first loaf of 100% whole wheat using the Tartine method.  89% hydration.  Gave the flour a 90-minute pre-soak before adding my leaven.  Four hours bulk ferment.  Four hours final rise.  Holes were a little tighter than I might like.  Any thoughts on how to coax it to open up a little better?  Longer pre-soak?  Higher hydration?

Thanks for any help you can offer!