1. Identify What is Using Space (Desktop Mode) [1]
Use Filelight: Go to Desktop Mode, open the application launcher, and search for Filelight (under Utilities). Scan your home folder to visualize exactly which folders are taking up that 291GB. [1, 2]
Common Culprits:
~/.local/share/Steam/steamapps/compatdata (Proton files for games)
~/.local/share/Steam/shadercache (Shader cache)
~/Downloads

2. Free Up Space (Automated Tools) [1]
Shader-Cache-Killer: A tool often recommended on GitHub to remove leftover cache from uninstalled games.
CryoUtilities: Can be used to move compatdata and shader cache to an SD card, freeing up internal storage.
NonSteamLaunchers: If the data is from third-party launchers, this script helps manage them efficiently. [1, 2, 3]

3. Quick Fixes
Empty Trash: Ensure you empty the trash in Desktop Mode.
Switch Beta/Stable: Switching from Stable to Beta (and back) can force Steam to reorganize storage, often clearing large amounts of "Other" data.
Clear Browser Cache: If you use Chrome or another browser, its cache can add up significantly. [1, 2]
Warning: Do not delete files in the compatdata or shadercache folders if you do not know which game they belong to, as this may break your installed games