Apologies for the typo. I took the command from my router and forgot to change before posting.
I don't think the guide you mentioned in your first post is correct for your circumstance. That guide uses DHCPv6 Prefix Delegation in which your router would send a DHCPv6 request to a delegating router. The delegating router would then assign an IPv6 address to your router. In your case the ISP has already assigned an address so you don't need DHCPv6.
I think all you'll need to do is assign an IPv6 address to the WAN interface and then add a static default IPv6 route to the gateway address the ISP gave you. For example:
set interfaces ethernet eth0 ipv6 address eui64 2a02:xxxx:1003:61::/64 set protocols static route6 ::/0 next-hop 2a02:xxxx:1003:61::1 commit;save;exit
Note that I've obfuscated the IPv6 prefix address in my post.
At this point you should see an IPv6 address on the WAN interface and be able to ping the IPv6 gateway address.
You'll then need to enable IPv6 Router Advertisement (RA) on the LAN interface so that your clients will get an IPv6 address via SLAAC:
set interfaces ethernet eth1 ipv6 address eui64 2a02:xxxx:1003:61::/64 set interfaces ethernet eth1 ipv6 router-advert prefix 2a02:xxxx:1003:61::/64
commit;save;exit
There are a number of options for the IPv6 RA, but the defaults should allow your clients to get an address.
As I mentioned I don't think you'll need the DHCPv6 PD configuration so that could be removed from eth0, as could rule 6 from the firewall.
delete interfaces ethernet eth0 dhcpv6-pd delete firewall ipv6-name IPv6_WAN_LOCAL rule 6 commit;save;exit
Regards