Hi Izella!
A big reason why most places need the likes of mumble is for fleet comms, where the FC and other command will be sitting in the top channel, and Squads in sub channels below. It means the squads can communicate with each other without cluttering up comms or the other squads and FC Command chat, and the FC can issue a command to all channels. Discord doesnt offer that functionality at the same level.
We may not be the fit you are looking for, however more than happy to chat (but yes, we use discord AND mumble)