As you noticed, I left the option to install on a hybrid and MBR from a system higher than MacOS 12, but your bootloader is saved in /Users/mbp151/Desktop/Legacy boot for Windows (disk6s1). Then manually copy to the Windows disk using third-party programs.@tomsmods @Sergey-Galan
I have attached the external drive to my MacBookPro15,1 running Sonoma.
Disabled indexingsudo mdutil -i off /
Unmounted manually the partition and Installed Window 7 in MBR mode.
Enabled indexingsudo mdutil -i on /