The soft poo could be due to the lousy food, and the switch which doesn’t always agree with a bunny at first. Or, she’s not eating all of her cecals and she is sitting in some of them, causing her to have a dirty bum. You can try to give her a half bath: put one inch of warm water in a set tub or sink. hold her wrapped in a towel, and sort of dip her back end into a set tub or sink with unscented soap or shampoo handy. You are better off not using the towel, but if you can’t get a good grip on her, don’t try it just yet until she’s a bit older and more secure with you.
I don’t think next week is too long to wait for the vet to do it for the first time. Call the vet and tell them you need it done, because they need to be ready.
Also, you can sprinkle some (unscented) corn starch, the stuff we keep in the kitchen, on her bum and back legs. It will soak up moisture, then the dried poo can be brushed off, or wiped, or just gently plucked off. It may come off more easily when she washes herself, too. It’s not a food, but it’s safe for bunnies to lick. She may need a little fur trim at the vet if she doesn’t handle a bath well. I often did a Butt Bath with the middle, front paws and head all remaining dry. But Spockie cooperated more than Sammy, who hasn’t needed one.