Thanks for the reply.
Yes, it is a rather small partition. I am running PipaOS instead of Raspian due to the speed of booting. By default it has a very small root partition, and I did not resize it much larger after creation. I only made the /home partition sizeable and did a symlink of /var/cache to /home/cache as the apt cache is normally sizeable, as well as /usr/share/doc ---> /home/doc as that's a sizeable one (as revealed originally by du -hs /* | sort -h)
Besides, after installation I reasoned that items on / probably won't grow massively - all temporary files should be dumped into tmpfs I am guessing?
Where abouts do the temp files typically get generated? I know /etc/.pihole is only about 4.7M (although /etc/pihole is 67M).