Discord cant stream desktop audio at all on Linux aside from sharing a tab’s audio if you’re using Discord in a web browser. There are custom clients (like discord-screenaudio which OP mentioned) capable of doing this to some extent but they’re based on the web version of discord and lack features / can be buggy. Also these options don’t have hardware encoding so any fast moving content will become a choppy mess for the viewers.
The other alternative on Linux is to just route the app’s audio into your mic source. Others will hear it but it will come out as if its your mic so even those not watching the stream will have to hear the stream audio unless they mute you.
Personally I don’t think I’d use it on my main phone as of now. While it does work well and the granular options for sandboxing/security are awesome, I’d worry about compatibility with banking apps and other more locked down software that, from what I’ve heard, will refuse to run under GrapheneOS. Also, yeah you’d be missing out on some of the exclusive Pixel features that are part of Google’s stock ROM. But overall it works well and handles Play Store backend stuff pretty seamlessly while still keeping the security tight.
From what I understand camera quality and other features may be less performant than stock as well, though in the case of my Pixel 5a the camera experience wasn’t that great before anyways so it didn’t matter to me.
For now on my main phone I’m just gonna keep using stock and dipping into Android betas to mess with the new stuff coming to Pixel phones. Maybe once my desire to be part of that ecosystem is lessened I’d just go for Graphene on my Pixel 7.