18h is the default on GrapheneOS IIRC. Got my phone stolen abroad days ago with tones of sensitive data and that features was a big reassurance. I set it to 6h I believe.

I tried to do it using Tasker on my crDroid device, but it requires the phone to be rooted, which mine isn't since some apps detect it and throw a fit.

I think you may be able to do something like this with a shortcut.

Iirc reboot requires user input so you can’t automate.

Yep. I tried to do it with a shortcut and it was basically a reminder to reboot that could be ignored.