I'm not sure you are justified in commenting on kernel stability with drives, when in your case, it is probably down to the interfacing you are doing, and not the drives themselves vs the kernel.
well E3, U2, M2 they are all connecting via NVMe over PCIe, it is just a different "plug", in fact most of the time the SSD use the same controller chip regardless of form factor.
and you are correct this system has no M2 slots
the SSDs are fittet via a passive pcie/m2 adapter.
system has even only PCIe v3 and one of the SSD has even only one PCIe lane attached.
but even so they are miles faster (latency!) and cheeper then attaching enterprise sata SSD's as special device

anyway i had the same symtoms as other people here, just wanted to throw in another data point that it also happens on different hardware and non ceph related usecase.
What are you running on that dual-core calculator?
mostly PBS sync target and a little PVE for homelab thinkering.
the calculator is 99% idle and still uses to much power for what it does
