One cool thing about GrapheneOS when I recently got a new Pixel 9 was how easy it was to install it with just my older Pixel phone. Because the installer is WebUSB based it works in the Vanadium browser. I just connected the two phones together with a USB cable and could install the OS on my new phone from the browser.
One currently missing thing is a "transfer" or backup functionality otherwise. There's really no good solution other than to manually port over applications and use their built-in import/export features if available.