Given the built-in updaters of Figma and Discord do not work when then application is not in /Applications, consider using the Homebrew project to manage the applications.
brew install --cask figma
brew install --cask discord
Once installed, the applications can be updated using the command:
brew update
See the StackOverflow question Changing homebrew-cask installation directories for altering where applications are stored.
Cache
To save more space on the internal drive, try a symlink for ~/Library/Caches to a folder on the external drive.
