Network drops almost daily. Rebooting pi-hole fixes it. What's wrong?

One thing to check, if the power supply isnt sufficient, "Under-voltage" can occur causing segmentation errors for processes and potentially even corrupt files on the SD card (see link below):

https://www.raspberrypi.org/documentation/hardware/raspberrypi/power/README.md

EDIT: The link changed:

https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#power-supply-warnings

You can check the syslogs for under voltage with below:

sudo zgrep -i voltage /var/log/syslog*

And check the kernel ring buffer logs:

dmesg -T | grep -i voltage

And check below one:

pi@phb5:~ $ vcgencmd get_throttled int
throttled=0x0