I'm following the documentation for setting-up the IPv6 config on the current 21.7 version of OPNsense. The documentation here: https://docs.opnsense.org/manual/how-tos/IPv6_ZenUK.html states that I should add a Gateway via the System: Gateways All but that doesn't exist any more, could some kind soul give me a clue as to where I need to put that entry - would it be in the System: Gateways: Group or is that only for load balancing?
I'm using Zen. Unlike your config I've got the WAN set up for DHCP (rather than going fully static). I've upgraded this system a couple of times, so when I set it up I was using the old instructions.
In my case, the gateway appears under System/Gateways/Single. Are you able to set up your static gateway under there?