There isn't much to m2 to PCI-e adapter cards. They do not convert the data, just the form factor. I use generic ebay ones for laptop mPCI-e to m2 and Vantec for the PCIe to m2. The generic ebay laptop ones are just PCIe x 1 lane and they still work fine. You'll still reap the efficiency and latency benefits.
It's possible to boot from nvme drives in systems that don't support via the BIOS. For EFI BIOS', I use Clover EFI. For non-EFI, I think what I use is called DUET. However, Clover EFI can work in non-EFI BIOS'. You boot into Clover/DUET EFI shell off either USB drive or SATA which then continues boot off the NVME drive.
Only issue I found is with non-EFI BIOSs in laptops is that resuming from hibernate doesn't always work unless the BIOS is in a 'shut down' state.