I have tested an S9 and it was full of preinstalled malware. No recent one but there is no reason to believe things have changed.

The USB port is disabled if water gets in. GrapheneOS used the feature to allow users to turn it off, always or while locked. That is actual security and it doesnt break the device it users want to use other (open source) operating systems on them.

LTE-only is relevant because 2G and 3G are insecure. They were part of the attack chain used by governments to install the Pegasus Spyware on phones, along with browsers allowing HTTP traffic and more things that are dealt with by GrapheneOS.

Google has implemented very few of their features to work on regular Android, riddled with proprietary spyware.

Dont forget

• your sources are really old
• they are very vague
• Samsungs mechanism is a middle finger for user freedom, that is well known and a reason why GrapheneOS cant support their devices
• if your phone has Meta spyware, samsung spyware, microsoft spyware and google spyware preinstalled, "military grade" (which translates to "good enough and cheap") is not relevant at all

Rethink is okay but buggy and thus leaky. GrapheneOS also has a network permission which is tremendously useful, also from a security perspective.

E.g. your keyboard shouldnt have internet access, or it might likely send all your input to their servers for "necessary analysis" lol