DNS Resolver is getting sluggish/unstable after time

You should avoid using a GUA address:
It's not stable, as its prefix is subject to change at the discretion of your ISP.
As the prefix changes, clients wouldn't be able to reach Pi-hole via its deprecated GUA anymore. Dual-stack clients will fall back to IPv4 after a while, which may contribute to your observation.

Use one of your Pi-hole's ULAs or link-locals instead.

You may also want to consider to prevent your router from propagating an IPv6 DNS server address as local DNS server to its clients altogether - provided your router supports it.

The FritzBox line-up should do so, see e.g. Unresolved ipv6 adress in my top list - #4 by Bucking_Horn.