Hi,
Welcome to ask here!
There should be at least 2 domain controllers and DNS servers in a single domain.
Since the DNS ad integrated in AD, both the 2 DCs should act as DNS servers.
The general recommendation is to not run any additional roles on your domain controller other than DNS.
So, the DHCP service can be added on the member server.
Or if you don't have so many servers, you can also put it on the DC, but should have failover set up on a second server.
DHCP Best Practices
No specific requirements for the second DC.
For the domain maintenance, you can also refer to the following method to back up DCs:
Back up and restore Active Directory domain controllers
This response contains a third-party link. We provide this link for easy reference. Microsoft cannot guarantee the validity of any information and content in this link.
Best Regards,