I fear any amount of clean install is not going to solve this, unless you want to make this a habit.
Pi-hole is not at the heart of this anomaly, it’s just suffering from it - and quite visually so.
Your Pi-hole dies because its logs spill over.
Even considering @jfb’s remark and restricting Pi-hole to log and display only A/AAAA-type queries would still mean you have well over 4 million requests of unknown origin per day.
Your above screenshot (post 3) shows that these requests amass during a certain time frame only - roughly between 11:30 and 15:00 hours.
Are you aware of any client device that is active exclusively during that time?
And as Pi-hole seems to see these requests as to be arriving from your USG:
Are you aware of any devices allowed on your network that do not use Pi-hole as DNS server directly, so they would be forwarded to your USG’s upstream DNS (that happens to be Pi-hole)?
Or alternatively, could your USG somehow try to act as an Apple compliant content cache?
And finally, to conclude with something more constructive:
As a counter measure, you could try and add those requests to Pi-hole’s blacklist as a regex:
On first sight, this approach is clearly inferior to @jfb’s proposal of logging only A/AAAA-requests, as blocked requests will still be logged, and thus still promote log file growth. However,
ANALYZE_ONLY_A_AND_AAAA wouldn’t make those stray DNS requests disappear - they just aren’t logged anymore.
My hope is that blocking them altogether will shy the clients -whatever they may be- away from repeating their requests. If this works as I hope, it would mean you won’t be able to take advantage of a content cache any more, but also that you can file a missing request report for 4 million DNS queries (only if you wish, though )