You got that right!
I started a new thread on Netgear's forum asking about using a non global unicast address and the first response was essentially "just use a global unicast address". 
From reading other posts in this community, I gather the main reason not to use a GUA is because it's not reliably static. The other reason I'm seeing is that it can cause privacy concerns. In this post, I think it's said that using the RPi's GUA makes pi-hole publicly available. I think I understand that second part a little less?
In the meantime, setting the link local address as the preferred IPv6 DNS server address on my PC's does seem to be working. It will be nice to find a fix for the router so it's working correctly for everything on the network.
Thanks again for all your help. I'll add an edit to the first post with the solution to the original Sophos question.