It’s not about proving it beyond a doubt, the OP’s frustrarion is about how the whole courtroom drama was inaccurate.
The assumtion is this: Legally, Starfleet only allows sapent adults to join its ranks. So a toaster cannot be a starfleet officer, and neither can a dog.
The fact that he is an officer means that in legal terms, Starfleet has already decided that he is a person, and any court that asks this question has a quick and easy answer.
As others have stated, grapheneOS is a good choice, and it’s extremely secure.
Another option is murena’ eOS. If you live in Europe, thier store sells phones with the OS preinstalled, and if you don’t, the easy installer app works pretty well with most models.
But fair warning, I recently installed eOS on my wife’s old Samsung S7, and I ended up having to do the installation manually with adb (command line tools). So read up on the phone you want to use, some are easier than others.
The operating system itself is great, and allows for anonymous usage of the play store (free stuff only of course) as well as F-Droid out of the box. You can also add your Google account to get full play store functionality, but then Google starts getting your info again, albeit less than with standard android.
Mozilla should approach proton to try and get accuired. I would love to see Firefox and Thunderbird become part of the proton landscape.