If you only have a single NVMe drive, it should always be in slot M.2_2 (on that particular board; most have the x4 CPU lanes on M.2_1 or M.2_2, but you always need to check the spec sheets or manual to confirm that). That slot is directly connected to the dedicated x4 on the CPU, whereas the others go through the chipset. If you have multiple drives, the system/primary drive should be in M.2_2. M.2_2 offers significantly more performance in some usage scenarios. M.2_1 steals lanes from the GPU slot, so should usually be avoided.
How long before a random reboot? Try testing the wd m2 in your old pc, while checking if the randoms stop on the new pc.
Agree Realtek sucks.
I'd avoid any rgb stuff.