You're answering your own question about why 10 months of evidence wasn't made public all the way through. If (and it's a low if) there's an appeal, it's not fair to Letby if she's had all this evidence put out to the public is it? People will have already made up their minds.
As far as the 'circumstantial' evidence is concerned, police look for patterns. Unless someone is caught with a syringe or something in their hands (and even that's explainable sometimes), it will always come down to patterns/perpetrator's ability to carry out the crime/elimination of anyone else.
For example, the issue around Letby's shift pattern. Other people were on duty when the babies died or were injured so could have done it if one or two babies were killed or hurt. But Letby was the only person who was present when ALL of them were injured. That's a pattern.
Yes, she worked with some babies who don't appear to have been hurt. But that could be her being interrupted, or not being able to get her hands on what she needed, a parent could have been in the room with another baby, a colleague may have offered to help or her motivation (whatever that was) wasn't there, or she may have just 'liked' that baby or the sun may have been shining that day. At the moment we don't know. We may never know. Or that could be in amongst the witness statements that we've not had sight of yet 'I offered to help her put the IV in' or 'I went in to feed my baby and Letby was there and we had a chat until her end of shift' or 'Dr X was on a two week holiday at the time' (if the alleged affair with the anonymous consultant was her motivation - if he wasn't on call, perhaps there was no motivation to do something?)
The Yorkshire Ripper didn't kill every woman he had contact with - only when circumstances worked in his favour. Same with Brady and Hindley - not every child in their town was murdered. Circumstances have to be 'right' and it can be a subtle thing that switches someone on or off.
I'm old enough to have lived through trials like the Yorkshire Ripper, Harold Shipman etc., I don't remember all the evidence coming out immediately after their trials in the press. Some of the evidence for them came out in subsequent documentaries or books years later.
On that point, the Yorkshire Ripper's trial lasted two weeks - because he confessed. Brady and Hindley's trial was about a month long. Harold Shipman's was about three months. Even Beverly Allit's trial lasted just three months. Letby's jury sat for ten months. That implies a lot of evidence. A lot.
The jury saw all the evidence in Letby's case, they were led through it by experts who could help them decipher what they were seeing. Yes, they may have been wrong, yes some evidence may have been withheld (as in the recent overturned rape conviction where DNA evidence wasn't put in front of his jury). But, at the moment, the jury are satisfied that the evidence they saw, the help deciphering it and the witnesses they heard was enough to prove guilt in some of those cases. To be honest, with the fact that the evidence has to be 'from the past' I'd have been more worried if she'd been found guilty on all counts. The 'not proved' type of verdict on the other babies shows that the jury have tried to be fair, looked for as tight evidence as they could get and not just gone for the jugular in my view.