Running installers, specifically very customized scene ones or repacks with heavy compression have compatibility issues with wine and proton out of the box. You can get the clean steam files from a .zip or equivalent, or run the installer on your windows machine then copy the files over via Warpinator, FTP or an exfat USB with a USB A to C adapter. Then add the games to Steam from desktop mode and set the compatibility to Proton Experimental and most games should work. If you transfer the clean steam files (of a game that is not cracked, give it the same name on the Steam store) of a non-Steam game you added it will say ‘Cannot verify license’ or something along those lines, so Steam can detect games you custom added steamapi.dll, you can’t verify the games files because it is not in your account so it cannot download the uncracked executable or steamapi.dll. I don’t know if Valve is actively scanning your non-Steam library and uploading that data to determine information about you. If you want to ‘de-steamify’ a Steam Deck you’ll still need a game launcher, there is opengamepadui, sc-controller for remapping and desktop layout. The biggest problem would be an on-screen keyboard. Maliit (a qt keyboard) is absolutely terrible compared to Steam’s built in keyboard, and the dual trackpad typing does not work with anything else.