I am coming to GrapheneOS from iOS, as recounted in https://blog.majid.info/quit-apple/
My experience with seedvault is not as positive as the OP, it fails for about half my apps. I guess I have another year to fix this when the GrapheneOS enabled Motorola flagship comes out.
I carry both my iPhone 16 Plus and my Pixel 8 Pro, but limit my iPhone use to mobile banking, Apple Pay and Find My.
Seedvault is godawful and I'm saying it as a someone using GOS as a daily driver. Utterly awful.
Two backup runs can give different results (failing in different places or failing to restore everything), and I haven't heard of the one person it works for every time.
I wish I could just run Titanium Backup :(