IPv6 & DDNS Semi-Technical

[Crimliar]

Using an Asus GT-AX6000 (Merlin Firmware), I've set up the ddns-start script to update an owned domain to point (DDNS) to both my WAN IPv4 & IPv6 addresses.
This works successfully most of the time but I've noticed that there have been occasions when the IPv6 address is updated without any change to the IPv4 address, when this happens the ddns-start script doesn't fire.  Obviously I can make the script fire using a new or existing cron job - but is there a better/more elegant way?

[countrypaul]

What trigger is being used to activate your script? If it relies on a change to your ip address is that by any chance ipv4 only? 

[Crimliar]

The documentation at this level is patchy to say the least.  When using the built-in (uneditable as far as I can tell) methods and selecting to update IPv6 the router loops (x10) as it tries to pull the address from ppp0, which is pointless as the address needs to be pulled from br0.  So you are probably correct and for some reason, the firmware is checking ppp0 for updates that will never happen.  As I'm using Hurricane Electric for the DDNS I don't need the router to know the IPv6 address to update it, so I'll probably just add an hourly cron job for my existing (working & simple) script.
I was just hoping that someone may have come across this before and had a more elegant solution!