Armcord is good and uses Vencord as well for plugins/theming. Webcord is another option that is primarily privacy-focused

Not sure if Vencord has a plugin that limits the apps telemetry itself though it does make using it better.
The harsh reality is that discord will sell everything you ever type or send thru it and there is no avoiding that even with 3rd party client.

No, but you could always bridge it to Matrix.

Webcord is what I use

I use vendetta but you can bridge discord through matrix