half a cup of butternut squash is about 7 gr carbs after fiber according to my counts book, so I'd sub out the squash too.

If you go back to the Atkins book and look at the allowable vegetables, you'll likely get great ideas for things to subsitute. Like notsosweet says, cauliflower is a great addition to soup. Broccoli is good too, but it tends to turn soups green.

If you want to "beef" that soup up a bit, but in some meat to add protein. Use chicken if you use chicken stock, or plain old water will do. So will cubed beef; cheap cuts simmered will get soft. Scoop out the meat before blending the vegetables.

BTW, squash has become my mainstay for soups. I buy the frozen cubed stuff because it is really quick and easy. In the winter I make a pot of soup every weekend. It's a Saturday morning ritual to get a pot of soup going. And I often eat soup for breakfast.

Good luck. You look like you are going to have fun experimenting!