i’d suggest starting by finding out what package in your distro actually decides where audio goes - mostly it is pulseaudio (older) or pipewire (newer).
depending on the details of how your distro and the dongle work, it could either be a simple “pactl set-default-sink <headset-name>”, or a more complicated set of udev rules or pipewire/wireplumber scripts.
note that distros using pipewire still often support a lot of pactl commands, so it may be worth looking at the simple option even when not using pulseaudio.
if it’s free, you’re the product.
signal seems really good right now, with open source clients, but they already show that they’d like to keep the ecosystem locked down by not allowing 3rd party clients. at some point they will need a way to pay for their datacenters, and even if they claim the foundation or whatever is doing well, i can see the pestering for donations getting much worse in the future.
that said, threema is far from optimal too, im still waiting for matrix servers to become solid options. last time i wanted to set up synapse, the only captcha they supported was fucking Google captcha :|