And yet many people on this forum have BQMs on IPv6 where the thinkbroadband FireBrick is pinging the user's router

.
As I did for over two years on AAISP.
I didn't mention NAT. You are the one not understanding the original question by the OP and the problem they had.
Here's
the latest example on these forums, a user on NOW. (The graph linked to has been deleted for the reason that becomes clear in that thread, where a later router-targeted one appears).They couldn't get an IPv6 BQM working precisely because the IP address detected by thinkbroadband was that of the device from which they connected when setting it up.
Turned that device off and use another two or three or many and the BQM became solid red. Turn that device back on and the BQM became normal.
If you want an IPv6 BQM you need to find the router's IPv6 address and edit the tbb BQM to ping that address.
Simples.
Here's
an older example with the method that I suggested here and also a generalised solution for BQM on IPv6 where the user has the more common single /64 or /56.
I was with AAISP. They allocate a static /128 address on their "internal" network to the user's router. For the customer's use they allocate a /48 block and withing that an initial /64. The customer can (or could at the time) then if they wished set up as many additional /64s as they desire. Excellent for businesses with many subnets. AAISP route all your /64s to the /128.
This is how we route and assign IPv6 on DSL connections.
Allocations
Customers are allocated a /48 block of addresses - this is usually per customer, and so a customer with multiple circuits or sites will have a /64 allocated from the larger /48 block. A /48 contains 65536 /64's and a /64 subnet is 18 million trillion addresses.
Logging in
(Here, Customer Premises Equipment (CPE) refers to the router belonging to the customer which is being used at their end of the broadband line, and L2TP Network Server (LNS) refers to our equipment at our end of the broadband link.)
When the CPE logs in and negotiates PPP, we use IPV6CP to negotiate an interface identifier - this happens at the same time as IPCP happens to negotiate V4 connectivity. Once this has been negotiated, the CPE should perform an ICMPv6 Router Solicitation - our LNSes will then reply with a ICMPv6 Router Advertisement in order to negotiate the Link Local address in the form of FE80::, and will be based on the MAC address of the interface. At this stage, there should be basic IPv6 connectivity to link-local, and if static routes and manually assigned addresses in place you should have a working connection. Our LNSes continue to send ICMPv6 Router Advertisements approximately once an hour (every 4096 seconds).
Optionally, if routes and addresses have not been configured statically, DHCPv6 can be used for automatic negotiation of the IPv6 WAN and LAN prefixes - using DHCPv6 is usually the default for our routers (i.e. the ZyXEL) that we supply. When DHCPv6 is enabled, the CPE sends a DHCPv6 Solicitation to the IPv6 broadcast address, containing a list of options that it requires. Once we receive the DHCPv6 Solicitation, the LNS you are connected to will send a reply containing responses to the options requested. The CPE can then take the options and apply them as appropriate.
WAN Address
In the DHCPv6 Solicitation, you would usually request DHCPv6 IA (Option 3, Identity Association for Non-temporary Address) to assign your IPv6 'WAN' address. This address is a single IPv6 address in the form of:
2001:8b0:1111:1111:0:ffff:[your IPv4 WAN in HEX]
LAN /64 Blocks
We also respond to DHCPv6 Prefix Delegation (Option 25, Identity Association for Prefix Delegation, responded to by Option 26, IA Prefix) from the CPE and we'll tell the CPE one of the /64 Blocks. -this may be requested multiple times by the CPE. A Client Identifier (Option 1) is also present in the DHCPv6 transaction, optionally DNS recursive name server (Option 23) can be requested.
At this point we will be routing any IPv6 blocks to your /128 WAN address. Usually a line will have at least one /64 block routed.
Customers can add additional /64 blocks on the Control Pages that will be routed.
If you want the whole /48 routed, untick the routing on all of the /64s you have, they disappear, and then tick the line routing on the /48 you have. Alternatively Support staff can help.
Your router will have to reconnect to AAISP before the new block is routed to your /128 WAN address. You will have to allow the new block through any firewall you have, and then route the new block appropriately on your local network.
Maybe you should tell AAISP and Sky support that they are not understanding how to use IPv6.
Connections: OnePlus 8 Pro on Three 4+ (LTE)/5G and at home Three Mobile, with (Three)ZTE MF286D router giving about 113/20Mbps.
1) Modern politics: The art of being the best liar.
2) There's many a slip 'twixt cup and lip.