My experience has been that it keeps well in a sealed container in the refrigerator for up to 2 to 3 weeks. Afterwards it gets "iffy" very fast. My experience with freezing 6 in 1, tho, had been so unsatisfactory that I wouldn't do it again. It seems to affect the feel, taste and character of the tomatoes in an undesirable way. Only experience can show that to someone, tho. For deep dish pizzas, I often add some small diced tomatoes (various brands) to give it a more chunky sauce.
Your idea of about 1 tsp of Penzey's pizza seasoning to 2 cups is pretty good. I vary that . . . sometimes more, sometimes less . . . as I taste the sauce on my finger with all the ingredients in it. I generally use it right away and don't let it "age" or set with the sauce, but that's an individual thing if one thinks it enhances the sauce. I don't.
To my 6 in 1 for thin crust pizzas, I also add about a half tsp of white pepper, a dash of ground ginger, roughly a tsp or two of minced garlic, a "pinch" of fennel seed and a "pinch" of Anise seed (both crushed with a spoon -- careful not to use too much as these are very powerful flavor enhancer's), a good dash of honey and sometimes a dash of olive oil. As this picture shows, I also experimented with adding a dash of coriander lately and that turned out pretty good. You can try various herbs and spices to see what you like. And, of course, this all varies depending on the size of the pizza.
Have fun deciding what's best for your tastes. I know there are many on this site who would say "less is best" and who knows . . . they might be right.
Edit -- Opps, forgot you need to salt to your taste, a tsp or so. I like sea salt the best, but it varies.