[NAME] You should leave the SATA mode setting at AHCI. As you found you cannot switch the SATA mode once Windows is installed on the drive without either reinstalling Windows or running a procedure to switch the mode of the Windows installation. This Crucial document recommends AHCI for a Crucial SSD. Here is an article about Secure Boot.  
I've been running for many months with the Crucial CT500P1SSD8 P1 500GB NVMe boot drive in the M.2 slot.  I recently decided to check the drive status by running Crucial Storage Executive. The app started to launch but immediately locked up the system with only the app logo displayed. I had to do a forced power off. Afterward the system would boot but was exceptionally slow - like a minute or more for each keystroke or mouse click.   I moved the NVMe to an external USB enclosure and it booted and ran just fine - normal high speed performance. I put the NVMe back into the M.2 slot and booted from a different SSD and everything ran fine including accessing data files on the NVMe.   I updated the Intel Storage Controller drivers and nothing changed. The NVMe drive works fine except when used as the boot device in the M.2 slot.   What could have happened during this system failure to cause this strange situation?  
XPS 8930, Crucial M.2 NVMe PCIe SSD, runs slow after a crash