Back up everything you can, but not using system backup features. Export your contacts and call logs if you need them. If you rely on Google's sync you can download their apps of course once GrapheneOS is installed. Try to back up your files too and if any apps have a backup or sync functionality make sure to grab a backup of those or sync them. Other than that you should be able to install GrapheneOS normally and manually restore your backups.
By the way, since you mentioned you're fully integrated into Google services: nearly everything works the same on GrapheneOS but keep in mind that Google Wallet does not, so make sure you have some kind if alternative before making the switch if you need it.