I have an 8 now that I really like. If you keep phones around forever its worth getting it because they will continue to support updates for 6 years.
Otherwise it really just depends on what hardware you like.

Here a install video and the Graphene page.
https://youtu.be/ywo-NT8K6B4?si=e23X4jG48sP936fH

https://grapheneos.org/install/