Prefix changes will be problematic

The solution as posted will fail the moment you have an IPv4 change on your upstream interface (remember that Comcast's IPv4 addresses are dynamic.) You need to bounce the IPv6 tunnel every time you get a new upstream address, and as such it needs to be invoked from DHCP (dhclient supports hanging scripts off events, so that's fine.) You *also* need the same scripts to update your firewall if you have any IPv6-prefix-sensitive rules, as well as update your internal DNS and DHCPv6 configurations (if you run either.)

Prefix changes, incidentally, are also expected to happen on a regular basis on the native IPv6 Internet, so expect the same kind of problems there.

Also, Comcast's 6rd configuration only hand out /64 prefixes, which means you either don't get to use RA (in which case you *have* to use DHCPv6 to propagate prefix changes to your network) or you can't subnet your network. 6to4 doesn't have that problem, but might not be reachable from the entire IPv6 Internet.