A lot of OnePlus phones come with MediaTek CPUs that don’t work with custom ROMs like LineageOS. So whatever you’re getting, make sure it’s using a Snapdragon. But for GrapheneOS, Pixel phones tend to be the recommendation so I’d just get one of those.
For Graphene OS there is no recommendation. It has to be a Pixel since Graphene OS only supports, for now, Pixel devices.
Good to know, thanks!