I was alluding to "max-load-1", "max-load-5", "max-load-15" in the linux watchdog daemon. These are system load limits.
The device timeout and polling frequency are something else.
AFAIK Raspbian has the device timeout set to the max value of 16 secs. This is a device module option, but since the device is compiled in the Raspbian kernel you'll need to
"modprobe configs" and "zcat /proc/configs | grep DOG" to verify that.
read the source code
So your watchdog program in linux should poll the device every 7 to 10 secs to be safe. If you set a write frequency of 16 secs in watchdog (watchdog-timeout=16 in the linux watchdog daemon) then you have a very good chance of rebooting suddenly. Use watchdog-timeout=10 instead, it should be safe.
I don't know the equivalent settings with the systemd watchdog, sorry.
I've almost never seen a linux watchdog daemon fail to manage a machine, even running on shoddy hardware. Check your configs.