You have enabled Conditional Forwarding:
*** [ DIAGNOSING ]: Setup variables
You may have created a partial DNS loop here, if your router had been configured to use Pi-hole as its upstream DNS server (as opposed to distributing it as local DNS server via DHCP). Pi-hole and your router may then bounce DNS queries back and forth among them forever or until time-out.
This would result in an excessively high volume of queries, which could explain high memory usage pointed out by jfb.
However, that is not confirmed by your current usage statistics, which show only a reasonably low amount of queries:
*** [ DIAGNOSING ]: contents of /var/log
-----tail of pihole-FTL.log------
Imported 23373 queries from the long-term database
-> Total DNS queries: 23373
-> Cached DNS queries: 5957
-> Forwarded DNS queries: 7176
-> Blocked DNS queries: 6395
-> Unknown DNS queries: 0
-> Unique domains: 862
-> Unique clients: 11
-> Known forward destinations: 5
But then, those may be low only due to a recent restart.
You could check your current stats (last 24 hours) by running:
echo ">stats >quit" | nc localhost 4711
The following command would give us an indication of the number of queries conditionally forwarded to your router in the last few days:
zgrep -c "to 192\.168\.1" /var/log/pihole.log*
You should check your router's configuration, and also check whether disabling CF would mitigate your issue.