As Ramhound noted in a comment, Secure Boot is not required to boot Windows. (Microsoft does require that computers are certified by them as Windows-compatible and that ship with Windows pre-installed ship with Secure Boot enabled, but that's a contractual requirement, not a technical one. You can disable Secure Boot and the system should boot just fine.)
Chances are you not only disabled Secure Boot, but also enabled the Compatibility Support Module (CSM), which caused the computer to boot in BIOS/CSM/legacy mode. EFI/UEFI-mode and BIOS/CSM/legacy-mode booting require different boot loaders and (for Windows) partition tables, so if you switch the computer's boot mode, Windows will stop booting. If I'm right, it was this change in boot mode, not disabling Secure Boot per se, that caused your problem. The description of your subsequent problems in your answer further supports my hypothesis; the complaint by the Windows installer that the disk was in GPT format indicates a BIOS-mode boot of the installation medium.
A better solution for your problem would have been to figure out how to boot with Secure Boot disabled but with the CSM also disabled, or at least to learn to control the boot mode (EFI/UEFI vs. BIOS/CSM/legacy). Unfortunately, details of how to do this vary greatly from one computer to another. Some computers provide clear and (seemingly) mutually exclusive boot modes; but on others, enabling the CSM provides merely the option of booting in BIOS mode. In some cases, what seems like a set of mutually-exclusive options is not so mutually exclusive; the computer might boot in EFI mode even if you set a "BIOS-only" option, or vice-versa. This can be extremely frustrating and confusing, particularly if you don't understand the difference between EFI-mode and BIOS-mode booting. See this page of mine for more on this subject.
As to your solution, if it works to your satisfaction, then fine; but be aware that a BIOS-mode boot may be a little slower than an EFI-mode boot. Also, enabling the CSM complicates the boot path, so if you decide to dual-boot in the future, it's imperative that you understand boot modes enough to ensure that you install your next OS in BIOS mode. (Mixed BIOS-mode/EFI-mode installations are possible, but are generally difficult to manage.)