Personally, I don't think that starting with one friend is enough to seal the deal with an anxious child. She needs to be more able to cope within herself than relying on a buddy to be there with her.

Obviously you know more about your situation and it is ultimately up to you, but you did ask for advice from those of us who have been in a similar situation. There is a massive difference between a 4.5 year old and a 5.5 year old. You wouldn't be holding her back, just not sending her early. All research I have read has pointed towards a later school starting age being better in the long run (e.g. Terman study). This is particularly true for anxious children, they do improve over time and it isn't worth pushing them when there is no real benefit to her being at school so early.
No, I agree one friend, even one she's had from her birth & through Kindy, isn't enough. It's a couple of things, like the school itself being small (220) with a track record of amazing supportive teachers for this very situation.

I know it's probably a cop out but it's good for me to verbalise this for me to process. As an adult with ADHD, I have struggled for the last 4.5 years to give her any semblance of a consistent routine, even when it comes to discipline. It hasn't helped her personality at all (where as DD2 is not affected at all).

I am so desperate for an enforced routine in the form of 5 school days because, despite all my planning and desire, I lack the ability to carry them through - to her demise. I have struggled greatly over the holidays and it's showing in her behavior.

So even though I know about the studies showing that earlier is no better, that later is more conducive to learning, and the fact that I spent over a year convincing myself she was going to repeat Kindy - she's going to start in less than 2 weeks.

No matter what, whether I sent her to Kinder again or Prep this year I am going to regret it - because essentially I would rather homeschool my school children but I don't have the capacity to