I can start having creamy soups in two days. Can I just make potato soup, but healthier? Is it ok to have salt and pepper and onions in it?

You will need to make it thin and pureed out the onions, no chunks.

You can also instead of using chunks of potato use instant mashed potatoes and then I'll make it more creamier and thinner!

I made mashed potato with broth and greek yogurt (for protein). Then, I pureed minced onions with the mashed potatoes and more broth, and it was yummy.

If you are going to make cream soup, I suggest you use cauliflower, rutabaga, brocolli, or other lower carb veg. Even pumpkin. Potatoes do nothing for you except slow your weight loss. I realize at this point you won't be eating much, but still, it's best to get into good habits early rather than later.