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.
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!