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.

You can also automate this with autossh which is designed for exactly this kind of persistent tunnel. Although a simple “while” loop might seem like the intuitive way to keep it running, autossh is very reliable and takes care of all the corner cases for you.