I don’t know what gave you the idea that a particular distro would be an especially good/bad choice for privacy, etc. They’re all GNU/Linux with only minor differences in compile-time options in the kernel and different defaults in user-space. But they’re just that, defaults. You can reconfigure them to your preference.
With that out of the way, the issue NixOS attempts to address is reproducibility. You get a central configuration infrastructure that defines everything, from partition layout, through user creation and package installation to software configuration. The central idea being that migrating to a new machine or setting up a new development environment should only take a few commands.
What you do with that is up to you. You can barricade the whole system if you like. The defaults are sane, but not overly focused on privacy, etc.
Also it’s quite a learning curve as the documentation/wiki is incomplete and/or outdated.
With the Fairphone you get more than just a replaceable battery. You get replaceable nearly everything. Also they do their best to ethically source the materials. In terms of ROMs there is also Iodé, also based on LOS, and if you go with a FP4 instead Ubuntu Touch.
Accidentally flashed a live image (PCBSD, IIRC) onto my 1TB external HDD instead of the thumb drive. Lost years of collected music and movies that night. I learned two things:
Don’t do this sort of thing in the middle of the night, when you’re tired and should be sleeping.
You can either try to contact the seller and ask for the password or just erase the UEFI settings by shorting some jumper or something. There should be instructions how to do that for your specific model.