Hey there! I asked a difficult and open-ended question myself; so I really want to try to respond to one as well.

Interestingly, Mozilla Hubs uses Elixir (and Phoenix) to power the network and a lot of the features you’re talking about: reticulum. Might be worth a look to see how they do things.

Regarding syncing users. I’m not sure what all this entails, but using websockets with Elixir is a full-duplex way of sending and receiving information without polling (and is the backbone of LiveView). Phoenix Channels and PubSub mixes into this too as the protocol for sending/broadcasting these updates. If you want to tie into it, I understand you’ll probably have to implement your own client in Go as outlined here.

I’m a bit of a noob when it comes to ECS here, so I’ll try my best. I don’t think you necessarily need to worry about the ECS pattern in Elixir. Use Elixir as your “network” for data updates like Mozilla does with Hubs. When new data comes into a client, store it in an ECS-compatible way and can use ECS there to make sure the new data is acted upon by your systems.

There’s quite a history of people using Elixir as the background of multiplayer gaming. There’s even a talk from the recent ElixirConf US talking about game dev in Elixir that might be worth a watch.