I think you answered your own question. So, unless you can get a higher temperature out of your grill, you may want to experiment with other possible solutions--such as oiling the unbaked rim of the dough with olive oil, pre-baking the pizza (undressed) for a short while and then dressing it and finishing baking, or using cold cheese, sauce and other toppings. The last two approaches allow for a longer bake time so that the crust can brown more completely before the toppings are done or the cheeses have started to brown. I have no idea if these "solutions" will work with your grill, so some experimentation may be required.
BYW, your recipe as you described it is a sponge-based recipe. I researched this approach some time ago and found references to it through Google searches. However, according to pizzanapoletana (Marco), sponges (and poolishes) are not authentic Neapolitan approaches. According to Marco, "Poolish or wet sponges have never been used in Naples for pizza. The only application for that technique in Naples is for a dessert called Rum Baba, of Polish origins. Poolish is indeed used from Rome to North, but NOT in Naples".
As far as San Marzano tomatoes are concerned, you will find wide swings of opinion on the subject. My personal overall favorite is the LaRegina DOP San Marzanos because of their high quality fruits and thicker than normal puree. The LaValle DOPs are also good and, in fact, that brand was the first brand of DOP San Marzanos I tried when I first started experimenting with the 00 doughs some years ago. At the time, my pizza crusts were also light like yours (my recipe was quite similar to yours but for the sponge method). I was using my standard home oven and the Bel Aria 00 flour (the Caputo 00 flour was not then available at retail), and I assumed that a light crust was perfectly normal. It took me many months to learn that to get authentic crust coloration I would need a very high temperature Neapolitan style oven. However, I enjoyed the pizzas with the LaValle DOPs, fresh mozzarella cheese, olive oil and fresh basil even though the crusts were quite light in color.
You are also correct in your observation that San Marzano seeds are available in the U.S. Although I have never tried growing the SMs from seed, I am fairly confident that you will get tomatoes that are of like quality to other good home-grown tomatoes. However, you are unlikely to get the same tasting product as obtainable from Naples and its environs because the authentic SMs are grown in volcanic soil. The French wine producers would refer to this unique and distinctive environmental characteristic as "terroir". As U.S. wine producers have discovered, it is possible to import the grape vines but impossible to import the "terroir".
Good luck with your aspirations to become a pizzaiolo.