Hi,
I am having the same issue and I'm in the same scenario as yourself.
I also have 2 OPi 5+ one with 16GB RAM and the other 32GB RAM. The 32GB version crashes as soon as the NVMe usage becomes intense. I have tested with different NVMe drives (both are 2TB size so... Not sure if it isn't some limitation on that side) and if I connect it through USB it works without a glitch.
I'm starting to suspect something's badly soldered or installed physically.
Did you manage to solve your issue?