I was adding mangle rules to the IPv6 firewall mangle section and it turns out there’s no way to mark routes in rules. However, in IPv6->Routes->Rules there’s an option for routing marks. What gives?
There is a “mark routing” in the IPv6 mangle actions, but it is only available through the command line and hidden in Winbox.
That being said, in most cases it makes more sense to use routing rules for policy routing, as it is easier to avoid creating routing loops.