5 years ago (nearly) the Horizon Issues Trial Judgement was passed down on 16th December 2019.Again I appreciate what you are saying but, (there's always a but). 4 years ago they had a specific allegation about one trial. I doubt this hearing started then. Why wait for the rest anyway ? Investigate the one you have clear evidence against.
There is a memorandum of understanding between the inquiry and the Met.
https://www.postofficehorizoninquiry.org.uk/sites/default/files/2022-08/POHITI MOU MPS.pdf