Broccoli, leafy greens, and peppers will not give you a good veggie broth (maybe the peppers if the are roasted first). Much too bitter! Avoid cilantro too or even any large amount of fresh herb or it will be overpowering. I find I have to lean carrot - tomato based to get a savory broth. The most important step-- I cannot stress this enough-- is to put the solids through a juicer at the end and add the resulting liquids back to the broth. If you don't have a juicer then wring the solids with all your might through an old moistened towel or t-shirt. This is my basic recipe for veggie broth:
1/4 cup olive oil
4 carrots, chopped
4 large stalks celery, chopped
1 very large onion (or 2 medium, or three small)
6 cloves garlic, peeled, left whole, crushed
1 28 oz can tomatoes
4 teaspoons (1tbs + 1 tsp dried thyme)
1/2 teaspoon whole peppercorns
1/2 - 1 tsp salt (to taste)
12 cups water
I caramelize the carrots, celery. onion and garlic for about 40 minutes beforehand in the olive oil (don't burn, just brown), then simmer everything for about 2 hours, turn off the heat, allow it to steep at least an hour, then proceed to the straining/ juicing stage. You HAVE to juice those solids to get an incredible broth, don't just throw them away. At the end you should less than a cup of pulp left to throw away/ mulch.
You can add a few bay leaves too but remove them before the juicing step.
Ps-- you said you had been saving veggie scarps for months in the freezer-- this is another problem. You should use fresh veggies for broth (other than canned tomato and dried herb) because frozen scraps might end up tasting like.... well, frozen scraps! You can freeze the finished veggie broth successfully in the freezer for a few months without losing taste, though.