Theres a lot on here about Seedvault, and backing up your phone. This'll be my thread where I share my experiences and tell you how I back up and restore my phone.
When I first started using GrapheneOS, I tried Seedvault. I bought a 64GB flash drive and attempted to back up to it. It always failed. So for 3-4 months I didn't do anything and didn't have backups. Then a little while later, I decided to give Seedvault another try. It worked... Okay. But not great. I used it for a few months but it still was unreliable and who knows if I could actually restore from it. So today I did something else that while takes some more time, at least gets me a more reliable backup. I went to Settings > Apps > All Apps > Show System Apps > Seedvault > Storage & Cache > Delete Storage. That reset Seedvault. Then I plugged in my USB drive and formatted it. I created a folder in it titled Photos. I then copied all my photos to that folder. Then I did the same with files, creating a Files folder. Then I created a Homescreen Setup folder. This contains screenshots of my homescreen so I could set it back up the same way. Then I used the app Applist Backup to back up a list of my apps. I put the backup file in a folder titled Applist Backup. Then I backed up individual apps and put their files in the respectively named folders I made. I also backed up custom ringtones and notification sounds. That completed the Owner Profile. Now for the Private Space, I used the same flash drive but just created a folder called Private Space. I then made another Appslist Backup folder and put the file in there. For my other profiles, I used the same USB drive and just made a folder with the profiles name, and within that folder made my Homescreen Setup folders and Applist Backup folders.
I plan on keeping this updated once a month. It may not be the most convenient, but it works. I will keep this updated to let you know how it works out, and when I get a new phone eventually I'll see how the restore process goes. I should be able to just plug in the flash drive and restore each profile. As for system settings, I'll have to do that all manually, but at least now I have all important stuff backed up!