Here's a quick tip for you on the Steam Deck: how to find out where all that drive space is going with a sweet program called Filelight. This will work great on Linux desktops too, since it's just a KDE application.
Available as a Flatpak package, you can easily install it from Flathub. On the Steam Deck (or a Linux desktop running Plasma), you can load up the Discover store and look for Filelight. You can also use Flatseal to allow access to other parts of your system, like an SD Card.
Filelight gives you quite a rather gorgeous and simple way to look through your storage, and find something perhaps you've left laying around that was forgotten. You can click inside each folder, and keep going deep until you see exactly where that space has gone.
Note: if you don't see it listed, you may need to add the Flathub Beta Remote using this command in terminal (Steam Deck has it by default):
flatpak remote-add --if-not-exists flathub-beta https://flathub.org/beta-repo/flathub-beta.flatpakrepo
For normal Linux desktops (not the Steam Deck), it's also usually available in your standard software repositories. Use whatever packing system you prefer.