FlipSid
When it comes to a complete phone backup, Seedvault is (currently) the only solution. I have no personal experience with it -as i don't have a need for a full backup solution- but many users describe it as poor:
https://discuss.grapheneos.org/d/6025-megathread-first-hand-experiences-with-seedvault
It seems that GrapheneOS wants to replace it with something new at some point; I am not aware of a timetable for this.
When it comes to synchronizing data, there are countless secure and privacy-friendly options, for which you can also get good information and help here in the forum.
Here are some solutions that are frequently recommended and that I can think of off the top of my head (not exhaustive):
Nextcloud, Davx5, Proton, Tuta, EteSync, Syncthing, Ente, USB stick
You can find more information in discussions like this:
https://discuss.grapheneos.org/d/15-how-do-i-backup-my-phone
And at the end of the day, you don't have to use none of them- if switching is too much trouble for you, with sandboxed Google Play installed you can also install and use all Google apps such as Google Calendar, Drive, Gmail etc.