Hello @ccoles146,
welcome to the Nextcloud community! 
if you don’t have IPv4 don’t add A record at all
this way you only test local connectivity
you only need port :8443 if you want to expose the mgmt interface to the internet - usually you don’t want this.
You didn’t state this but I have the feeling you are talking about “port forwarding” and NAT when you speak about “opening ports”. This is not how IPv6 works. Each device has valid public IP and there is no more NAT with IPv6 - you simply tell the router/firewall incoming connection to a specific device will be allowed (using interface ID) - I shared how to expose service with IPv6 on fritzbox in the past. look at topics tagged ipv6 as well