Curious, why do you look for alternative due to Microsoft buying Discord if you need it for Windows anyways ? Also While Mumble and Element/Matrix are fine alternatives, issue is common people do not use them. At best some game groups do use Mumble or Open-source / Free software people might use Element, but very few… not big user bases.
I just saw article about Mailfence who “teamed up” with Thunderbird. And first look at their website looks good to me. Edit you have them listed lol