Post says, he has a 2012 5,1. If he is booting of NVMe now, means he has to have bootrom 144.0.0.0.0. So OS should be Mojave at least.
...and in this case he does not need OpenCore nor installing Catalina (which i would still strongly recommend to avoid on a productively used system) to get things set up. Mojave’s Disk Utility should be more than sufficient. Why make things (a lot) more complicated than they need to be?
Only question is, if he prefers performance (which means Raid 0) or data security (which means Raid 1).