It can be automated by adding it to the DNS template. We do it this way, and it is working perfectly. For us, we have always used a host (us) level dmarc. We set it to a specific email address which we can easily search and sort for any issues.
In using DMARC records since they were first recommended, we have had exactly one customer request that the dmarc be specific (email addresses) to their domain.
These days the DMARC is becoming more essential, so I believe it is better to setup the generic one with our email than to not have it. If a customer needs it tailored to their needs, they typically already know what is going on, and will just modify it.
Go to Settings | Platform | DNS Zone Templating
And set it up there.