One thing though is that I found them too chewy or leathery to my taste. Even others commented on them that they taste good and provide a good jaw exercise I wonder if they were too thick and please let me know how can I better adapt bread flour to NY style so they won't end up being too chewy or leathery...or that's how it's supposed to be?
Pretty much by definition, a NY style pizza will have a chewy crust. It is also sometimes described as "leathery". Usually the first line of attack to reduce the chewiness in a finished crust is to use a weaker (lower protein) flour. In your case, in lieu of moving down the protein scale to all-purpose flour, you could combine some all-purpose flour with the bread flour to achieve a final protein content that is somewhere between the two flours. A handy tool to use for this purpose is November's Mixed-Mass Percentage Calculator at http://foodsim.unclesalmon.com/.
You don't have to limit yourself to using all-purpose flour to soften the bread flour. You can also use pastry flour or cake flour. However, the more of these flours you use, there will be some loss of overall crust color and flavor.
You can also soften a NY style crust by using oil and/or sugar or honey in the dough, albeit at the expense of moving away from the NY style, which classically uses no sugar or oil, to arguably some other style. But, if oil is added to the dough, depending on its amount it can help retain moisture in the dough during baking, thereby leading to a softer crust and crumb. Sugar and honey, by virtue of being hygroscopic in nature, will help retain moisture in the dough, even while the pizza is cooling after baking, and thereby contribute to a more tender eating characteristic. Both of these options (oil and sugar) are commonly used with pizzas described as being "NY style" even though that characterization will be contested by some. A point to keep in mind when using sugar or honey is that if the dough has too much sugar (or honey), you perhaps don't want to bake the pizza on a very hot stone surface. You might be able to use a pizza screen on the stone surface to raise the pizza above the baking surface during baking and thereby help minimize excessive bottom crust browning. You can see some examples of NY "thin"/American hybrid pizzas at this thread: http://www.pizzamaking.com/forum/index.php/topic,1707.msg15310.html#msg15310.
A still further way to soften a finished crust is to cool the pizza on a substrate that will force some of the moisture escaping from the pizza back into the pizza. For example, if you cool the pizza on a metal serving platter, instead of on a metal grid or insulative cardboard round, some of the moisture trying to escape the pizza will go back into the finished crust and help keep it soft. I often experience this effect while I am trying to take photos of pizzas cooling on my metal serving platter.
Depending on the dough formulation, no doubt there are baking methods and temperatures and bake times that can help produce a less chewy crust.
There is no reason why you can't use your Ischia starter culture with a bread flour, or even a high-gluten flour. I did some experiments along these lines with the basic Lehmann NY style dough formulation and got good results.