I just found this cool utility a couple days ago and have found that it's a great solution to sandboxed versions of Discord being unable to provide rich presence. It's a Python script that queries the Steam web API to get info to forward to Discord's API. The "installer" sets up a systemd service that will auto-run the script for you.

It requires you to get your own Steam API key which was a trivial, though slightly buggy, process for me. You can optionally use your own Discord app ID, or just use the one owned by the guy that created this script.