Is there a best practice method for updating multiple AAAA records when/if the /64 prefix delegation I'm getting from AT&T changes? I've read that an A6 record used to be in place for this sort of thing but near as I can tell it's now obsolete.
One option is to update the AAAA records using a dynamic DNS client / API call.
https://dns.he.net/ supports dynamic AAAA records. The homepage has examples.