As to why the judgement is very long: Ordinarily cases before a court will have a lot of agreed facts. For example maybe Mrs Smith says Mr Jones knocked down her cherry tree with his car and Mr Jones denies this, the two sets of lawyers will meet before court and agree that OK, yes, this blue car is Mr Jones' car, the cherry tree belongs to Mrs Smith, it's on her land- and so on. Then the court gets to focus on a real dispute.
The agreed facts don't take the court's time (the judge will read them, but he doesn't need a court in session to do so) and only appear in a judgement if the judge happens to mention them for some reason. But for the most part this court was obliged to tediously "find" things that should have been agreed up front. Since the plaintiffs are human and so have finite financial resources and lifespan, whereas Post Office Ltd is a corporate entity wholly owned by the British Government we can reasonably believe this was a deliberate stalling tactic for which a judge ought IMO to have sanctioned them.
The judge stops short of this, concluding that "Over the numerous hearings and two full substantive trials that I have conducted, I have gained the distinct impression that the Post Office is less committed to speedy resolution of the entire group litigation than are the claimants, but it is not possible to state with finality whether that is correct"
An example the judge brings up early in their writing is that the plaintiffs say the Post Office did this to other Sub-post masters but eventually fixed it and paid them back. The Post Office says it didn't, but, under cross examination actually Post Office staff admitted that yeah, obviously they took money from other sub-post masters due to bugs, but they considered that since they'd paid it back months or years later that meant it wasn't taken at all. With a few hours haggling over the wording this could have been an admitted fact both sides agreed on, albeit one that makes the Post Office look pretty bad, but instead they insisted on wasting the court's time.
The Post Office doubled down on the stalling tactic, first trying to get the entire case thrown away by arguing that this judge was "biased" because he had found against them. They asserted that they were entitled to a judge who basically believed anything they said, what the purpose of such a judge could be is unclear, but this wasted yet more court time. They also hired new lawyers who insisted that they ought to be allowed to start over because they (the new lawyers) hadn't been there to make better arguments earlier in the trial...
Basically there was no plan beyond "Stall and lie and hope all the plaintiffs die before we lose". A competent government might have observed this, terminated the Post Office CEO for gross negligence and ordered her replacement to settle years ago.
Also notable: Some plaintiffs have Criminal convictions for their supposed "theft" from the Post Office as a result of shortfalls in Horizon. The Criminal Cases Review Commission is charged with considering situations where people are found guilty of some crime as a result of gross miscarriages of justice - most often they're "fitted up" by corrupt cops, or expert witnesses at trial insist upon something subsequently proved to be factually wrong. The commission will have wanted to wait until the civil case is completed before considering whether any of these plaintiffs were wrongly convicted and should be sent back for a new trial (at which a court might conclude that with the current evidence there was no crime and so their conviction is overturned automatically).