The best thing to do is just start with whatever you already have. An old gaming rig, laptop or Raspberry Pi, doesn’t matter. Each will present some technical obstacle that you need to overcome (for example keeping the laptop going when the lid is closed). That’s part of learning.
Self-hosting is a hobby where we gradually learn more. Experimentation is just as important as reading the docs.
Gentoo is fun and a nice way to learn more about computers. Their wiki and their community was really good when I was into it, I’m sure it still is. But compiling everything from scratch is quite demanding of your CPU and your time, so it’s not really something that you run as your daily driver for long.
Also lots of killers seek psychiatric help voluntarily (and are often sadly ignored). For a sane, moderately competent person it’s easy to plan the perfect murder or terror attack – it’s a different thing to carry it through because a sane person also has mental guardrails.
Mossad is effective because Israelis are ruthlessly trained to dehumanize anyone who’s an obstacle to their goals.
It’s great that smart people are working on this, but I don’t think we can expect hobbyists to make a useful OSS implementation of smartphones. Especially since there is so much dependence on the hardware. We either need a company that can throw some weight behind it, or just straight up governments that value it (e.g. from a sovereignty point of view).