No additional functionality is available yet to perform these types of functions. Most likely these types of features will be available in 2020 once new features get released.
The only work-around that you have would be to install some screen capture software along the lines of DU Recorder or something like that which can capture video on your phone screen and then edit/save/publish etc. Not ideal, but about the only option available currently.
It doesn't help logging into Stadia via Chrome from a PC either as screenshots and clips are not displayed there yet (although you can take screenshots and generate clips, they can still only be displayed via the Stadia app not via Chrome).
Yes! I would love to be able to post to Twitter, and it would be cool if we could post to our favorite Discord channels too. Also let us use the screenshots as wallpapers for our favorite devices.