Prevent Google Play from reinstalling apps after factory reset
If you've already allowed Google Backup to backup your apps, then changing settings before you do the factory reset won't make any difference.

After the factory reset, when you next start the device, it'll go through the first-time setup steps again: where it guides you through connecting to a Wi-Fi network and signing into a Google account. On the screen where you sign into your Google account, there's a checkbox there to automatically restore your apps and data onto this device (I forget the exact wording).

Uncheck the checkbox at this point, and no apps or data will be restored to your device. You'll still be able to install any paid apps you previously bought. If you later turn on Google Backup on this device, it'll start a fresh set of backup data. Once it has backed up to this fresh set, next time you factory-reset this device, the restore will be from this fresh set. (If you factory-reset again before making any changes, or before it's had a chance to back up, you'll still get the old set.)