The commands have confirmed your RPis to have different public IPv6 addresses, which you could configure in your router.
With IPv6, you should consider a few things before deciding on an address (click for more)
You are currently using EUI64 IPv6 addresses for public traffic, which exposes your devices MAC address to the public in the interface identifier.
Using an RFC 7217 IPv6 address would hide your MAC while still having a stable IPv6 interface identifier. You may control this by editing your /etc/dhcpcd.conf to contain the following line:
slaac private
A stable IPv6 address is still tracable, so you may feel tempted to enable IPv6 Privacy Extensions (RFC 4941) on top of that as well.
However, when considering an IPv6 address for Pi-hole, you should probably avoid using a public address ( 2000::/3 range) altogether: Not only is Pi-hole not meant to be publically available, but with a public IPv6, both your IPv6 prefix and the interface identifier are subject to change (the former by your ISP, the latter by IPv6 Privacy Extensions and the likes), and Pi-hole requires a stable address.
Consider to Use IPv6 ULA addresses for Pi-hole (fd00::/8 range).
If that isn't an option, try using your Pi-hole's link-local IPv6 address ( fe80::/10 range).
You can list all currently assigned IPv6 addresses for your RPi's eth0 interface with the following command:
ip -6 address show eth0
Whichever address you decide for in the end, configure your router accordingly.