I also hate auto updates, have a Motorola, and the same issue.
After much grief and aggrevation, I stumbled quite blindly onto a sort of solution.
I have a certain cleaning app, quite possibly the only trustworthy one, that can be set to automatically clean the device once a day.
After I allowed the auto cleaning to occur, the automatic restarts stopped!
My guess is that although the auto update is turned off, the junk still gets downloaded and will pester either you or the system itself until it gets installed.
These things are installed automatically during restarts, so there it is.
So what I figure is happening now is that the cleaner is deleting the update data soon enough after it comes in that my device has suffered only one or two restarts in the last few weeks- whereas it was happening multiple times a day before.
Try it and see if it helps you as well as it did me.
https://f-droid.org/en/packages/theredspy15.ltecleanerfoss/
Now,
@ocnbrze has a valid point about the age of the device.
This app may help, but the fact remains that you will need a newer device shortly.
You very well may have the same issue when the updates come through on a new device, especially if you turn off the auto-updates like I do.
So, even if you replace the device, I would still put this app on the new one and let it do its thing.
When you read the description, you will see why I say that it may be the only trustworthy cleaner around.
Also, if you don't know, F-Droid is the most secure appstore around- so don't be put off that the link is from a third party appstore.
