Just because Discord doesn’t allow third party clients doesn’t mean none exist. For instance, there is purple-discord although to my knowledge no libpurple-based app exists on Android yet. My solution is to run this through bitlbee (IRC gateway) and Quassel distributed IRC client, with Quasseldroid on mobile. It requires a lot of setup though and requires an always-available machine, so might not be ideal. It really would be simpler if there was just an app.
A Libre federated protocol like Matrix is the idea, but replacing a proprietary app with a Libre alternative is always a win even if you’re still tethered to the proprietary service. One step at a time.
edit: an alternative, if you control the Discord group and can add bots to it, is to run a bridge bot that connects a Discord channel to an IRC or Matrix channel. Again, though, this requires that you are able to have a machine that’s always on.