Dear M.,
Your poor little girl....
Fortunately, my children never had problems with UTI's, but I surely have.
Your daughter may have a fear of restrooms, etc, but my guess is that she is also afraid to pee because it's very painful when you have an infection. I mean, it HURTS. The only thing that hurts worse is when the infection goes into the kidneys as well.
Somehow, you have to help her understand that our bodies filter out things that aren't good for us and it comes out in our pee, so it's really important not to hold it.
Be sure she is getting enough water and try to get her to drink cranberry juice every day. Maybe get her a little plastic 3-minute egg timer, the type that looks like an hour glass, and have her be sure to sit on the potty until the sand runs out. It will give her something to hold and look at and might distract her enough to relax and get her bladder emptied. That's something she can keep in her pocket and use even at school. It might help her get into the habit of not rushing through going pee.
Another thing I suggest is to have her start taking showers instead of baths. Since she is so prone to UTI's, sitting in bath water is most likely not good for her. Make sure she wears good cotton panties. Wipe from front to back. And again, get her to sit still long enough and often enough to keep her bladder empty. Those are all just habits she might as well get into now.
I hope the ultrasound on her kidneys doesn't show anything to worry about there.
Best of wishes.