Very much appreciated! 
The two you mention seem to be working well. [1]
Getting it to work with a Fritz!Box isn’t that hard, once you’ve figured it out. I’ve added a screenshot of my setup. Maybe it is of any use. If you are interested to find out how much of your NTP traffic is IPv6, you may like this little tool I wrote.
Although I am naturally very happy with any/your additional IPv6 capable servers and welcome them, there is this other thing that the NTPpool maintainers should do, which is even more important at this moment. And that is that they should change the DNS of the NTPpool, so that not only 2.pool.ntp.org hands out AAAA addresses, but [013].pool.ntp.org as well.
Please note there is more ways to accomplish this. You can do it in one go (internet access via IPv4 and IPv6), or in two separate steps, depending on your specific situation.
Also, obviously a fixed address is required (just as with IPv4). Quite often ISP’s suddenly hand out a different IPv6-range and/or different IPv4-address, which can spoil the fun.
[1] 2a00:7b80:477:21::9dcb:d174 seems to have some issues from some sites, but I would have to check closer to be sure. UPDATE: tested it from the NLNOG-ring and everything looked fine!