Hello MattUA,
Thank you for posting in Microsoft Community forum.
It looks like your AD environment with Backup Exec, NPS, DNS, DHCP, CA, DFS, SQL BAG failover cluster, and Splunk is more complex, it is recommended you would probably just create a new domain and move or migrate everything to new domain using ADMT tool if needed.
Here is a similar thread with detailed information for your reference.
Internal Domain Rename - Microsoft Q&A
I hope the information above is helpful.
If you have any question or concern, please feel free to let us know.
Best Regards,
Daisy Zhou