MacOS – Creating a bootable Sierra image on Catalina

I received a MacBook7,1 a few days ago and wanted to use dosdude1's Catalina patcher to install it on my system. The first attempt actually yielded a result that constantly showcased weirdness. When I looked at the page again, I noticed that he recommended having upgraded the BootROM through Sierra. Now, the MacBook did indeed come with Sierra but I had already flattened the SSD to put Catalina on it instead. And now, I am well stuck.

Trying to use createinstallmedia from the Sierra installer app's resources folder does spawn an uncountable amount of processes, but does not actually ever ask me to erase the drive and tell me that it's copying files over. And any other createinstallmedia that I had available to try just outright did the same or refused to accept the application image.

Then I thought of booting up a Sierra VM. After all, I did have the image already, so installing it into a VM and using the createinstallmedia tool from within there should yield me a usable result. But no – just dropping macOS into a VM won't work at all, even if the host is a macOS either.

I have tried several methods, but I am quite out of ideas by now. I may also have a failing harddrive, but using the latest GParted Live DVD to load the Smart Monitor GUI only had me tell me that everything was fine with it. Weird.

How can I create a Sierra image on macOS Catalina (my other, 2014 MacBook Pro) that I can boot off on my MacBook7,1? Alternatively, I have a Windows 10 PC available, so if there is a method to make such image on Windows, this would also help.

Small bonus: Do you happen to know why createinstallmedia just basically throws out so many other createinstallmedia processes?