I'd advise against setting static IPv6 addresses on device, unless you are sufficiently familiar with IPv6 and fully aware of the associated consequences.
For your Pi-hole machine, try instead to set a fixed IPv6 interface identifier for that device in your Fritzbox.
Be aware that such an interface identifier only ever gets assigned for those clients requesting it via Stateful DHCPv6. Most clients won't by default, and some never will (e.g. Androids).
See also Pi-Hole IPv6 - Hostname resolution noob questions - #15 by Bucking_Horn, where you asked almost the same question.
In general, don't change setupVars.conf by hand.
Pi-hole's installation, update, repair and reconfigure scripts will read and write information from and to that file.
To reconfigure Pi-hole for changed network settings (i.e. a changed IP in your case), run pihole -r and choose Reconfigure.