Matrix and its implementations like Synapse have a very intimidating architecture (I’d go as far as to call most of the implementations somewhat overengineered) and the documentation ranges from inconsistent to horrific. I ran into this particular situation myself, Fortunately this particular step you’re overthinking it. You can use any random string you want. It doesn’t even have to be random, just as long as what you put in the config file matches. It’s basically just a temporary admin password.
Matrix was by far the worst thing I’ve ever tried to self-host. It’s a hot mess. Good luck, I think you’re close to the finish line.
Climate change is absolutely a now problem, it’s affecting far more people right now than acid rain ever has or ever will, it’s costing trillions and you’re already paying for it in taxes, insurance, and other ways. The fact that people don’t understand it or appreciate how significant it is doesn’t mean it’s not a “now” problem, it just means it’s not a problem that’s ever going to get fixed because people like you don’t and won’t ever consider it a “now” problem. Enjoy the challenge of struggling your way through the increasingly impossible economy, it’s not going to get better.