Problem came back again while I was working on the exit.
unbound-control purge_requestlist
does not help but it appears that
unbound-control purge_infra unbound-control purge_requestlist
will clear up the problem without requiring a daemon restart--at least temporarily.
Also tried setting
do-tcp: off
but this did not appear to make a difference.
Seems to me a degenerate interaction between tor's 'eventdns' subsystem and 'unbound' comes into play when this DNS flood/attack occurs. Have an 'info' level log with SafeLogging=0 for a few minutes where the relay was in the bogged-down state and was failing to service Tor Browser requests. If developer is interested in taking a look at this please contact me directly.
This issue is a PIA and if it continues I'll give up on 'unbound' and follow the previous operator, switching to bind9 despite the lesser performance.