xIUPITERx I'm pretty sure it won't work the same way on GrapheneOS.
What will happen is that after official support for the device is over, it will go into extended support release mode (it's unclear if this will happen from 6th gen and beyond, as those devices already have 5 years of official suppport), in which you'll continue getting AOSP updates until the next Android version, when extended support releases usually stop.
Either way, with or without these Google Play system updates, any device past its official EOL date can no longer be considered secure, so you should choose a device that provides enough of a lifetime, such as 6th or 7th gen.
You can find information about device lifetime here:
And the project's current device recommendations can be found here: