There's a lot of advice floating about the Internet on configuring IPv6 on Xfinity/Comcast networks, but none of it applied 100% to my local situation. I therefore wrote up what worked here with RouterOS 7.14.

While I welcome advice from the IPv6 gurus here, do keep in mind that what you see in that article was arrived at after much careful experimentation, striving for the minimum set of changes to the default configuration to get everything working.

The configuration depends on if you have mid split enabled or not. It looks like you do not have mid split enabled yet (yours looks like my old config for the default route). Once mid split was enabled in my subdivision, I had to set the default gateway to the interface name. This is most likely tied to the new vCMTS that are now in use with mid split.

I doesn't surprise me at all that my working configuration is contingent on local NAN-scale engineering decisions.

(NAN = nuke-area-network. Smaller than the Internet, bigger than a typical WAN.)