James80 if you don't mind me asking, why do you need re-install GrapheneOS?
If you factory reset your phone if will go back to GrapheneOS fresh install, so there's no need to re-install it as such.
If you really want to go ahead with resetting it, please do not rely only on the Seedvault backup facility in GrapheneOS. It can be quite unreliable (GrapheneOS devs are currently working on a replacement). If you do go ahead, make sure you also copy across your all your files manually. Many apps also allow you to do manual backups too. You can save these files on your USB drive too.