As Pi-hole isn't meant to be publically avalable, you should avoid using a public IPv6 address (2000::/3 range) for Pi-hole. Also, public IPv6 addresses are more likely to change often, as both the prefix and the interface identifier are subject to change.
As stable address assignment is mandatory for Pi-hole to be reachable by your clients, you could consider using a ULA address (fd00::/8 range), provided your router allows; see Use IPv6 ULA addresses for Pi-hole (that topic is linked in the debug log as well).
If it doesn't, you could try and use your Pi-hole machine’s non-routable link-local IPv6 address (from fe80::/10 range).
Note that devices need to be within the same network segment for this to work (commonly, that's all devices connected directly to your router).
Any L3 switching will prevent devices connected through such a switch/router/AP to communicate with Pi-hole’s link-local address.
In general, you shouldn't edit setupVars.conf manually.
Instead, run pihole -r with Reconfigure.