Yeh the Nova 5s are such an oddity. Only had one pair, first 200K I loved them, got to 300K and they were full on done, dead, awful. I've read a lot of similar and polarised views, reckon it has to be a quality control issue.
RE. Carbon Plates - I've done my fastest runs by far in Vaporfly 4s but there must be some placebo effect thrown in there too I reckon.
Been looking at the New Balance 1080 v15s, they sound like what I need but I've zero experience with NB and the colourways stink (I know that's only minor).