I made this yesterday turned out well! Good way to use zuchini. I didn't have any celery seed (local grocery stores out too) and used a half cup of chopped celery. I am not much for the sweet relish (Wife and kids like it though) I am thinking maybe today I will try another batch, but leave out the sugar. Have you tried it without sugar?
If you don't like sweet relish, I wouldn't leave the sugar out of this one. It is meant to be a sweet relish. I would go with a Picalilli or green tomato relish. The problem with changing recipes for canned goods is that the balance of sweet and acid is important so it doesn't spoil in the jar.
I'm glad your family liked it.