Earlier quoted context omitted.
I think my PiHole is up for 3+ years on a Raspberry Pi dedicated to that task. Did not fail once since then, so not sure if "DNS is going down" is really an issue. But maybe I've got survivorship bias.
Living in a North American city with power wires being above ground, I have had so many power outages in the last five year, it was kind of a crazy thing to get used to. My Pi would not deal well with power outages when running through the SD card and so I stopped using it.
I also regularly reboot the pi by simply cycling power.
The solution was fairly simple. Send the linux log files to /dev/null (or whatever it is actually called, i.e. RAM) and disable query logging in pihole.
That's it. Helps greatly!