The default settings are designed to get online with most ISPs that use DHCP and otherwise comply with standards. Some of the settings that you have changed in lan make no sense. The default ip6assign 60 will work with an ISP that issues a /56. You could change it to 64 if you will not be running additional routers downstream on the LAN.
Prefix delegation has nothing to do with reaching the Internet from the router itself though. For that the important thing is the default route received via RS/RA.
Can you ping the default route target? It is probably inside the cable modem (but if the cable modem is a true bridge, it could be at the ISP at the other end of the cable). If the cable modem is doing any routing internally, it must of course be properly configured.
Note that to ping a link local address it is necessary to specify the interface: ping fe80::xxxx:xxxx:xxxx:xxxx%eth0
Many cable companies lock to the MAC address of your router. If you change to a different router MAC it won't be able to reach the Internet unless that lock is released. In some cases that can be done by turning the cable modem off and leaving it off the network for at least 30 minutes.