In a food processor coarsely chop tomatoes, zucchini, sweet peppers, onions, and garlic. Measure 8 cups chopped vegetables. Place in a colander to drain excess liquid. Transfer to large nonmetal container; sprinkle with pickling salt. Cover; refrigerate 16 to 24 hours.
Transfer vegetable mixture to a colander; rinse and drain well. In an 8- to 10-quart pot combine vinegar, sugar, thyme, and pepper. Bring to boiling, stirring to dissolve sugar. Stir in vegetables. Return mixture to boiling. Remove from heat. Can or freeze as below.
Ladle hot relish into hot, sterilized half-pint or pint jars, leaving a 1/2-inch headspace. Wipe jar rims and adjust lids. Process in a boiling-water canner 5 minutes for half-pints or 10 minutes for pints (start timing when water returns to boil).
Place the pot in a sink of ice water; stir relish to help it cool. Ladle relish into wide-top freezer containers, leaving a 1/2-inch headspace. Seal, label, and freeze up to 10 months.