You can probably use an Elgato Facecam (Mk.2), configure it in a Windows VM and then save the settings directly on the device to use it under Linux.
Lemmchen
Does this support other targets like twitch VODs?
Nah, that was public opinion on /r/datahoarders (I think) back when I still used reddit.
People complain that they won't respect their lifetime plans forever, but otherwise I haven't heard anything bad about them.
Never had an issue getting an iso from them via Mullvad.
Mattermost does not have E2EE to my knowledge.
Not E2EE though, or at least not fully.
XMPP only does it with certain client extensions. And Matrix only does it when the rooms are set up this way. SimpleX does what you want, but is kind of unintuitive for the average user.
I say go with Signal, it does what you want and is idiot-proof.
But as a third party you can not know which clients are using this outdated http mirror. On BitTorrent you can see every participating peers and some of them are probably enduser machines (depending on the actual implementation of OP's suggestion).
https://gitlab.futo.org/keyboard/latinime
Debatable how "proprietary" it really is.