(Not a plumber) but is it possible the cylinder PRV is faulty / set to too low a pressure so mains cold feed pressure forcing it open and pushing out all the warming water?
Also not a plumber, and I haven't even read the whole thread, but this is the Internet so they're not prerequisites for wading in anyway...
Recalling that you've had your first hot shower for a while I was wondering if your observations have all been shower based? Do you get hot water out of a hot (non-mixer) tap? Just wondering if the shower feed is imbalanced such that the higher pressure cold supply is preventing the lower pressure hot from getting through.
As I'm writing this I'm feeling it is a low value contribution given the great minds already in play, but again in accordance with the rules of the Internet I won't delete it but post it anyway...
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