We tried hiding the KMSI option in Company Branding, however received a 'Failed to create company branding' message and aren't certain why. This article suggests, 'Your domain name must appear as “Active" in the Domains portion of the Azure portal in which you have configured branding.' Do all domains have to be verified in order to configure company branding for the first time, or just your primary domain? Any other ideas why hiding KMSI would fail?
I'm pretty sure you don't need to have all domains verified, in my tenant I have few "test" ones that stay unverified, but haven't spotted any issues with the branding yet. @Kelvin Xia might be able to help here, or at least know who is responsible for the branding. Or you can open a support case.