if you can, buy an unlocked phone. Some carriers like to mess with things that might prevent you installing Graphene. I haven’t heard any recent specifics if Version is a problem or not.

Export app configurations for any app that lets you, it speeds setting up the device a lot.

Assume no data will properly be transfered (as a percussion), so have backups of everything you care about. (I forget if Graphene even has a transfer feature?)

Graphene’s stock software leaves a lot to be desired. I like the Fossify apps for phone, contacts and files. For texting, I stick with google messages, its the only one I know of that works right with everything. For camera, I love Open Camera. For the Home launcher, I like lawnchair.

There’s many options for app stores; ideally you pick and one or two, having more than needed adds complexity. I stick to fdroid, play store and graphene’s “store”. Graphene’s app store pretty much only has their core apps and what’s needed for compatibility with the play store.