GrapheneOS would be in control of the client which is what matters in an e2ee system.

Not really... You will be communicating with other people who don't use GrapheneOS and if you are e.g. communicating with iPhone users who have iCloud backups enabled (most iPhone users) and did not opt in to ADP (against most iPhone users), the chats are only encrypted at-rest in the iCloud backups and are accessible by Apple and law enforcement.

Signal is way better because it opts out iCloud backups (and Android backups) in favor of truly end-to-end encrypted backups.