finally managed to get everything working again.
Involved erasing and reformatting the system drive, re-installing everything, removing several 3rd party extensions (LaCie Raid & Wacom), re-installing the Highpoint driver, then still getting nowhere…
Was getting really annoyed that both Recovery and Safe modes could see the nvme, but not the standard boot mode. Eventually found a newer driver on Highpoints site that installed and allowed the nvme to mount!
10 mins later an email from Highpoint tells me about the newer driver, but also some interesting info highlighted in red…
“We have had feedback from customers that the driver does not load when using the controller in Mac Pro 2019 in Slot4, if your SSD7140A is inserted to slot4 we recommend you replace it to slot3 or 5.”
Mine is currently in slot 4 so as soon as its finished the backup i’ll be swapping the slot to a different one.
edit: Thanks Jan, Sam, Waldi and BenV for all the help and advice on working through this problem.