Best Practice to replace External DNS to F5 GTM

No real best practice but do have some different implementation approaches. The one to use will depend on what you are trying to do and willing to have all DNS queries, including none Wide IP queries, be handled or traverse a F5 DNS system.

In the following guide see the sections

• Replacing a DNS Server with BIG-IP DNS
• Placing BIG-IP DNS in Front of a DNS Server
• Placing BIG-IP DNS in front of a Pool of DNS Servers

https://techdocs.f5.com/kb/en-us/products/big-ip-dns/manuals/product/bigip-dns-implementations-12-1-0.html

https://support.f5.com/content/kb/en-us/products/big-ip-dns/manuals/product/bigip-dns-implementations-12-1-0/_jcr_content/pdfAttach/download/file.res/BIG-IP_DNS__Implementations.pdf