Some Lemmy instances don't federate correctly but most of them do. Be wary, it takes time - I thought it wasn't working for a bit.
The kbin matrix space is pretty helpful, if you have a matrix account.
Other considerations:
The MERCURE_JWT_SECRET .env needs to be 32 characters long (duh, but I wasn't thinking when I was setting passwords)
The .env file gives an example media subdomain as media.your.domain but it should actually be your.domain/media
Installing yarn might actually install something totally different. The docker instructions get around this using some custom repository in the instructions. We got around that a different way by installing the latest version of nodejs, npm, and installing yarn from npm. (We are using Ubuntu 22.04 so the experience might be different for you)
We are rapidly running out of space on our VPS and can't diagnose it. We know it's not the amount of media stored. The docker containers steadily increase in size until the instance hangs and we have to restart, but at this point restarting still consumes 80% of our 80GB of space so we have to diagnose this and fix it.
I don't know how much propagation time is involved in synchronizing instances. Most of the time it seems pretty quick. I'm seeing the OP plus one reply on lemm.ee anyway.
I wouldn't even say it's kind of referencing Star Trek. The term "federation" was coined way before the creator of Star Trek was even born. Federations have existed for centuries before Star Trek was a thing.