> Which is very much contrary to software freedom
I believe you misunderstand what "software freedom" means. You can compile and install GrapheneOS yourself, and you can grant yourself admin access. This is software freedom.
Software freedom does not mean that you should run everything as an admin, always. And just in case: software freedom does NOT mean that you should remove your firewall and let everybody SSH into your server by having a blank password.
You can't grant yourself admin access with the official build. Only the Graphene devs have the ability to push changes to the OS on your phone. Yes you can fork the software and build a version with your own signing key, then wipe your phone and install your custom build and thereby take back control, but then is that really still Graphene?
I think it's fair to say that that's at least borderline anti software freedom, even if it's true they have good security reasons for doing things that way.
Thinking about possible ways they could retain the same security properties without impinging software freedom... maybe there's a way they could make the root of trust default to a signing key embedded in the device's own secure hardware? Then by default that key could sign Graphene's own signing certificate to allow them to push updates, but the user would retain the ability to revoke that signature and sign someone else's certificate instead (or their own certificate) if they decided they didn't trust Graphene anymore, or wanted to give themselves root.