I knew of Grayjay but did not know it was open source now. A lot of the app is written in Kotlin which is promising for cross platform

Cross platform

None of the are available for both mobile and desktop

It can be cross platform. On mobile and desktop

True. I did make a feature request to have the Newpipe Extractor in the app

https://github.com/mu-fazil-vk/FluxTube/issues/44

Discord is the worst. Requires a phone number, does not allow email aliases and logs your chats.

Matrix and SimpleX is way better