Yes, you are correct. For anyone else in the same position re questions, I would suggest reading/re-reading the first post as
@tsialex does a very good job keeping detailed info up to date!
An ignorance-based question: when people say that "upgrading to XXX bricked my mac", is the reason for the failure always in relation to the bootROM/flashing as opposed to the OS install itself? And technically speaking, is it an electrical failure of some sort, like frying the bios chip? Or is it that the firmware is somehow incompatible with the system hardware, and so the computer is not electronically dead but simply unable to load/execute the bios to start up? Is the age of the computer/whether it is "officially supported" also a risk factor? For example, the risk of flashing to the most current bootROM would be 3,1 >> 4,1 > 5,1, or does this have nothing to do with it?
Also, [for getting NVMe boot drive to work] what is the difference in terms of flashing from an official, full installer for Mojave vs injecting an NVMe driver into the firmware? Is one safer or better vs the other? What are the advantages of doing one instead of the other?