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

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.