Looking at your Debug i see very little to concern me or make me think there is something amiss with your setup.
Which version of Raspberry Pi is this on?
You could change blocking modes within your FTL config to NXDOMAIN as the NULL blocking mode forces us to store all domains twice, once for IPv4 and once for v6. Blocking mode - Pi-hole documentation
With that many domains being blocked, there is an increased memory load and you are using a Pi with the least amount of memory (I recall that model has 256K {Edit - 256M} RAM).
There is not a memory leak, just a large memory load.
Just a small note that this will be fixed with v4.2 when NULL will use exactly the same amount of memory as NXDOMAIN does right now. Due to more testing that needs to be done, this will unfortunately not be released with the upcoming v4.1.