As much as it is a controversial use. IPv6 masquerading is ultimately needed for mwan3 but as you say NAT66 is the only option when you don't have a prefix at all. I have seen NPT6, but in my case it won't work for all my upstream WAN connections.

The reason for suggesting 2.8.6 is due to an issue with 6in4 tunnels being constantly marked as down, due to mwan3track thinking this is the case. It's due to an issue with being able to test via the interface name vs the specific GUA /64 on a tunnel interface. A hack was implemented in this version for it, I'll clarify that statement however.

2.6.18 suggests you are an older OpenWrt build i.e. 18.06, but if you aren't using 6in4 IPv6, then that's probably OK. I’ll clarify the specific mention of this to clear up confusion.