Scot, I think you've gotten some good suggestions.
One thing that I would add is that the quantity of sauce and the hydration of the sauce both impact foldability. Neapolitan pizza can have soupy issues if you use too wet of a sauce and/or too much of it, but, I think, in your case, in order to facilitate a more tender undercrust, I think you could use a bit more sauce.
How are handling the dough on the stretch? Are you being gentle? How many hours between balling and forming?
If I had to pick one potential culprit impacting tenderness and foldability, it would be the acid generated from the starter. Dialing in the right amount of bacterial activity can be a tricky process. Normally I recommend that beginners use cake yeast/IDY first, then graduate into starters, but, you are obviously not a beginner. I still think, in order to get a baseline texture, it might be worth doing one or two batches of dough with cake yeast.