How To Back Up Active Directory System States

The better method for high availability and disaster mitigation is to maintain at least two domain controllers. If the PDC / FSMO role holder fails you can seize roles to remaining DC, perform cleanup and rebuild failed one without downtime.

https://support.microsoft.com/en-gb/help/255504/using-ntdsutil-exe-to-transfer-or-seize-fsmo-roles-to-a-domain-control

https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2008-R2-and-2008/cc816907(v=ws.10)#bkmk_graphical

Otherwise it is recommended to take, maintain, test regular bare metal backups.