To add some context to other people here, you are applying to join the collective with your structure, RollenSpielMonster. Currently, you are providing a voice call service named Teamspeak that is proprietary, which will probably block your application, so you have ~1 or 2 month to find a solution or you may apply later to the collective.
I am not part of the collective either, but it seems that using FOSS software is a hard requirement here. To help you find a solution, you might ask yourself why you want to join the collective and also why Teamspeak is so important to you. Also understanding why people would be tempted to migrate to Discord could be interesting.
In your post, you dismiss Mumble because, if I understand correctly, 1) no one knows Mumble and 2) it is hard to operate (from a sysadmin point of view?). Not knowing a software is not really a problem, not wanting to switch from one software to another is. Still, I think none of these problems are insurmountable. For part 1), you now have a good reason to ask your users to switch to Mumble: your application to this collective. People here can give you some explanation on why FOSS is a hard requirement. For part 2), if you have specific questions or problems, people here might be able to help.
Having a better understanding of what are your use cases, what are the important features to you, what are all the blocking points, etc. would be useful to us. Also having an idea of the size of your user base could also help us to give you some tips on how to start the discussion with them.
Also, depending on your needs, but if you have some programming skills,
gstreamer is a nice libray that has everything in it to create a voice call application (and a lot more!). For example, I created a small CLI VoIP client in ~300 lines of C for my PhD thesis. On the theoretical approach, on your client, you need a jitter buffer algorithm, the audio codec « OPUS », an audio mixer, and maybe encoding your frames with the RTP protocol to help your jitter buffer (all these components are in gstreamer). The server can be as simple as relaying the data it receives to all the other members of the group. You may add TLS for privacy. If you have very specific needs and some programming knowledge, it can be a path that you may want to follow. It seems you are into role playing games, it would be a great opportunity to develop a software specifically designed for role players.
I have also seen the project fosscord.com that seems to be in early beta/alpha that you might want to test. The only concern I have is that it might encourage people to switch to Discord by familiarizing them with the interface while pushing them away from the FOSS server due to its bugs/missing features.