Actually you should have at least 2 hosted accounts for emergencies:
https://learn.microsoft.com/en-us/azure/active-directory/roles/security-emergency-access
and the global admin should not an account synced from on-prem as you alluded to.
You can remove the first account as a GA, but why?if anything, leave it as one of the break glass accounts and create a new one hosted one for yourself that you MFA with
Also, which accounts are the billing admins? the same as the first GA?
You can certainly remove that first one, but I see no reason. I would make it a break glass along with another and create a new GA hosted account for yourself protected with MFA.
Just my two cents :)