You won't have any accounts to sign in to anymore unless you install an app that requires an account. You are free from forced accounts that track you.
Graphene is very barebones at start. You need to use the web browser (Vanadium) to install an app store to download apps or download apk's straight from the source/Github.
You sync data by either manual backups or by using your chosen app for syncing. Syncthing, NextCloud, ProtonDrive, Ente, Immich, etc.
I would say that the fdroid store is better than the PlayStore. Everything is open source, no ads, and a lot less/close to zero junk apps spamming my searches. You also have access to repositories to add even more apps.
I would suggest looking at DivestOS instead of Lineage. It's more secure as you can re-lock the bootloader. I do suggest Graphene over all else for you specifically as it's incredibly simple to install and the Google Sandbox works very well.