Going to try using the flour one more time. Will reducing water increase strength? The dough seemed very wet, perhaps over hydrated.
Generally, dough strength should not be a focus if you want a tender crust as a result. Flour protein/gluten, kneading time/technique, along with fermentation time, temperature and humidity can all affect "strength". For a Neapolitan dough, I would recommend using a hydration of about 58% when you're just starting out. Instead of "strength" as your focus, Focus on getting everything to mix thoroughly and get the dough smooth. Don't overknead. Just enough to get it smooth. As you gain experience with your dough making and handling, you might want to experiment with higher hydration percentages until you find what works for you. However, beware that climate and environment changes and will affect dough, so there really is no 1 recipe-fits-all unless your in a restaurant environment with very controlled temperatures and the dough never sees any changes.
Most importantly, get yourself a little notebook where you can keep track of your dough adjustments and note the results. This will prove invaluable for you in the future!